Strange jaw movement? Help, please!

Hi! I'm very new to leopard gecko ownership. I've never owned any sort of reptile before, but I heard these guys are easy to keep. I've done as much research as I could prior to purchasing my leopard gecko. When I arrived at the store, I asked a store associate about the housing of their leopard geckos. I asked a question about how the crickets were just left in with them because I heard that was potentially harmful, and she then informed me that if I wanted a leopard gecko, they just had two new arrivals earlier that day. They weren't on the sales floor because they were too young and small to be housed with the older ones. She brought the pair out and I selected the one that seemed more docile. The other vocalized when being handled and that freaked me out a bit. I figured that the baby would be a better choice since it hadn't yet been subject to poor pet store conditions. I could be wrong, but I'm just trying to lay out my thought process for you guys. Anyhow, here's the questionnaire thing!
About your leo:
- Sex: Unknown.
- Age & Weight: Unknown. He's a baby, though. About four inches long from nose to tail tip?
- How long have you owned your leo: Five days
- Where was he/she obtained (ex. Pet store, breeder, wild caught, friend): Pet store. I know, I know. But I couldn't have one shipped to me.

A) Health/History
- How often do you handle your leo: Only let him walk across my palm once. I want to give him time to adjust.

- Is your leo acting any different today? If so how does he/she normally act which differs from now: Same as always I think. One minor change is that we purchased a hollow bonsai tree thing for his hot hide when we got him. There's a single branch on that tree and after his first night, he found a way to climb up to the branch from the inside and he liked to sleep there for the first three nights. Kind of like a loft for himself. I thought it was cute. The past two nights have been spent not in his branch but just in his hide. He also sleeps outside on the hot side of his tank. I figured this behavior just meant he's not too scared of us anymore?

- Has he/she had any problems in the past, if so please describe: Nothing that I know of
B) Fecals
- Describe (look any different than normal): His first few bowel movements were small and hard to notice. The one this morning is about 1.5cm in length. Half is white, half is black. It looks normal compared to the "healthy stool" article I read. Hah.
- When was the last time he/she went: This morning or sometime last night. I woke up to it.
C) Problem
- Please briefly describe the problem and how long it has been going on: This literally just happened this morning. He was laying on his stomach and relaxing and my roommate came in and removed the lid to his tank. She brought over his crickets and calcium powder and he turned away from her and proceeded to make a strange motion with his mouth. He opened and closed it five or so times somewhat rapidly like a silent bark?

Housing:
A) Enclosure
- Size: Ten gallons
- Type (ex. glass tank): Glass tank
- Type of substrate: Slate tile
- Hides, how many, what kind: Three hides, one on the cool side, the fake bonsai on the hot side, and a wooden log thing for a moist hide in between. I use damp paper towels in there.
B) Heating
- Heat source: Under tank heater
- Cage temps (hot side, cool side): 70 on cool, 85-95 on hot.
- Method of regulating heat source: I don't really have one. I just monitor it.
- What are you using to measure your temps: digital thermometer with probe
- Do you have any lights (describe): no lights. We have a moonlight bulb we intend to use at night once we have the proper lamp for it.
C) Cage mates
- How many (males, females): None
- Describe health, or previous problems: Nothing

Describe Diet:
A) Typical diet
- What you're feeding (how often, how much): Around five small crickets a day
- How are you feeding (hand fed, left in dish, ect): Hand fed? I drop them into his enclosure when he comes out. I figured he might see me as less of a threat if he watches me dump food in there.
B) Supplements (describe how often): No supplements on the first two days, but all crickets have been dusted in Rep-Cal phosphorus-free calcium with D3 for the past three days.
- What vitamin/minerals are you using (list brands): Rep-Cal
- What are you gut loading food with: Fluker's Orange cube. I was considering fresh fruit as well? But I dunno.

So that's about it! I'm very new at this but I'd like to do it right. Thank you in advance for any advice you can give!

Difficult to picture quite what he was doing. He could have been startled and trying to scare her off?

Reading over your setup, most of it looks just fine. Things I would like to comment on:
- No thermostat is generally not a good idea. Most UTHs are capable of topping over 100 F. You said you are measuring with a probe. Is the probe laid flat on the floor inside the hide? Just checking. I know you have slate tiles, so maybe that will keep the ground from overheating, but the fact that he's lying up high or out in the open instead of inside his hide concerns me a little. Just want to make sure it's not too hot in there. Ideal floor surface temp is around 90-92 F.
- If you only have calcium with D3, you will also need a reptile multivitamin to dust with a few times a week. If you like Rep-Cal brand, they make Herptivite vitamins. Most people also recommend a very small dish of pure calcium without D3 be left in the cage at all times for the gecko to lick, especially for babies. They can need more calcium when growing, and it's difficult to get the right balance by just dusting.

Besides those comments, everything sounds good. I highly recommend feeding your bugs some healthy fresh veggies and fruit as a gutload. Nothing beats fresh nutrients. See this list of the best gutload foods for ideas:
Much Ado About Chameleons: The Anatomy of Gut-Loading | Ingredients & Nutritional Info

Thank you for the response! I've been a little freaked out about it all day but you might be right. I'm usually the one to feed him so maybe that was it. I'll let you guys know if he does it again. He opened his mouth very wide about 5 times in a row in rapid succession. I dunno.

As for the tips, thanks a ton for the help! Can you elaborate on the reptile multivitamin? One of the care guides I read used the two terms (Calcium with D3 and multivitamin) interchangeably so I didn't realize that I needed more than that. I did intend to get him plain calcium for his enclosure but they were sold out in the pet store nearby. I'll go back in a few days. I'm also looking into getting a thermostat! I'm scared the pad might malfunction or something while I'm in class. xD I worry more often that he's too cold than too hot, though. His temperatures usually hang closer to 85. The probe is on the floor directly next to his bonsai. Should I stick it inside so I know what the temp is like in there? That's where he spends most of his time. I'm not sure what brands are good or where to start on the thermostat. Also, if you or anyone else can offer me some more tips about babies in particular, I'd reeeally appreciate it! Most things I've found speak in reference to juveniles and adults. D: I'd also like to handle him... how soon is too soon? Sorry for the slew of questions. I just really don't wanna mess him all up. Haha

A multivitamin is a general reptile vitamin supplement with a lot of different vitamins and trace minerals, sort of like a human's daily multivitamin. Calcium + D3 is just those two things and doesn't have all the other vitamins that the gecko could need. That's why you should have both and alternated dusting the food with one or the other (or mix the two together).

To be honest, I really love Repashy Calcium Plus, which is an all-in-one dusting supplement that is multivitamin, calcium, and D3 all together. ZooMed also makes an all-in-one called ReptiVite, but I have no experience with that.

Thermostats are a great investment. Some lower-cost brands that are still reliable are HydroFarm and Big Apple Herp, both of which you can find online (like Amazon.com). For probe location, you want the temperature probe stuck right flat on the floor in the center of the heat pad where it will be the hottest. That spot should range around 90 F. It will tend to go up and down a little as the thermostat cycles on and off, but it should stay 88-95 F.

Babies are skittish and flighty little guys. Handling really isn't recommended until they get larger. This should be a time for the gecko to get used to you and your hands being around without the hands chasing him too much. You can lie your hands in the cage while feeding him, etc., just to show him you aren't a threat. As he gets bigger, you can try moving your hands closer, touching him, getting him to walk across your hands in the cage... it's a very gradual process.

Alright, awesome! I'll just buy some of that online so I'll be sure to get the right thing. I live in a college dorm and the mail room is a little slow, though. Is the multivitamin something I need right away? It'll probably take 5 days to get here.

I'll order a thermostat, too!

My gecko's name is Puck, by the way. As far as I can tell, he's more inquisitive than skittish. I tried putting my hand in last night and he climbed right over it! Tentative steps but he didn't have to touch my hand at all. I tried again this evening and he stood on my fingertips while drinking from his water dish. He let his tail rest on my hand. I've never touched a leopard gecko before. He's the cutest little thing! Anyhow, I promise not to rush things despite being super excited.

I can't seem to remember the last question I had... But seriously, thank you so much for the help!

It's okay, I don't think he's going to get malnourished in a week. Just make sure the bugs are gutloaded with healthy foods and use the calcium+D3 for now.

Great that Puck seems used to hands already! That's half the battle. Some of them are wicked scared and run to hide from hands. Hopefully, he'll warm up very quickly then. Keep an eye on his reactions. If he seems unhappy or stressed, back off a bit and start over.

Anyway, just post if you have any more questions. It can be a little overwhelming when you first start, but these guys are really easy to take care of once you have the basics down.

Just a fun update! Puck is doing great. My roommate was a little impatient about the handling thing, but lucky for us, he loves it. He'll approach and climb onto hands when they're put into the tank and gladly climb all over us when he's taken out. Such a curious and adorable little friend. Thanks again for the help! No weird jaw movements after that day. He just yawns occasionally.