If you put a bunch of healthy fleas into a jam jar which they can can easily jump out of, then put on the lid, the fleas will keep jumping up and hitting their heads.
After a while, they'll jump just short of the lid because they don't want to get a headache. If you take the lid off, the fleas won't be able to jump out any more.
There are two ways to get them to jump out: either put a Bunsen burner underneath, or put in a normal flea which jumps out straight away and the others will follow it.
People are either motivated by force or by inspiration.