Okay, the top one, when it fits correctly, pretty much stays up thanks to suction. You pop it in, and it might feel a bit slidey for the first few mins, but the suction works up and it stays put. That's the best I can explain it. If you're worried about it falling, you can use a little of the special adhesive (the stuff is magical, ain't no denture movin' if you've used some of that). But a properly fitting top denture will stay up on it's own. My immediate was loose for a while, but the adhesive allowed it to stay up.
The bottom one is more tricky. It doesn't get the suction, so it tries to move around a lot. Your mouth adjusts to this and learns to control it - but again, adhesive can work if you're struggling.
You shouldn't need adhesive for properly fitting dentures, so don't worry about having to use it. It just might help, either if your denture is a little loose, or if you want the added security.
