A lot of times, people want to get a snack, though sometimes it is in a jar of some kind. When they try to grab for whatever is in the jar, they will get their hand stuck because of the way that they have a grip on the thing in the jar.
This is a Comedy trope in which when you grab the contents of a jar, your hand winds up getting stuck, because of the way you have a grip on it.
The difference [...] [tvtropes.org]