Fleas are placed in a glass jar which is covered by a transparent lid. The fleas try to jump out of the jar but obviously hit against the lid. After 3 days left like that, they become so used to jump to a certain height that they will never be able again to jump higher even despite the fact that they still want to break free. It’s just an old PS2 advertisement so I don’t know if all of this is real, but if it is, well, it’s very interesting, if it’s not, well… the ad is still cool ))