1) A good fit will allow saliva to form a thin seal between the denture and your palate giving it 'suction'
2) Control of the denture via muscles of your tongue and cheek to stabilise it in place
3) Lower teeth helping to stabilise the upper denture in place
It's a good question, conne, which J9PUR has answered as our resident dental person! - but yes, as long as the top plate is a good fit, it stays in place with suction from your saliva.
When I only had one tooth on a little plate, it could often fall out - now I have more, it's much more stable.
If the gums shrink a bit then the top plate will become a little looser which is what happens to most people with dentures,
I rinse my top denture clean, dry it with a paper tissue and then add a couple of spots of Fixodent adhesive to the top denture,
I then rinse my mouth with cold water and finally pop in the top denture.
Drying the top denture before adding the Fixodent assures that the Fixodent adhesive stays on the palate when it is removed and does not remain stuck to the roof of your mouth like a dead slug.
No problem.
The dentures are only as good as the dentist and the dental mechanic who makes them. It's just a case of getting used to them once you have them, there's no going back but at least there's no more pain if they fit well. Any pain caused by the dentures can usually be corrected.