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 ))

It's what you don't see that is frightening!

Look especially the second half of the video, there’re some insane moves…