There were a bunch of really boring, scientific differences between monkeys and apes, but the easiest way to tell them apart is to just look at their butts.

Monkeys have tails, apes don’t have tails.

This one seems so played out in movies.  The nerdy scientist/animal-lover guy always reminds the no-nonsense military boss that they aren’t “monkeys,” they’re “apes.” 

For instance, an animal escapes from the zoo with top-secret government info:

FBI Chief:  So you’re tellin’ me the stupid monkey just walked off with the briefcase?!

Scientist/Zoo-Keeper:  Actually, it was an ape, sir.  And they are very smart…

FBI Chief: I don’t care what it was.  Find the damn thing!

They do it with dolphins too:

FBI Chief:  So you’re sayin’ the stupid fish just swam off with the briefcase?!

Scientist/Zoo-Keeper: Actually, it was a dolphin, sir.  They are mammals and very smart…

FBI Chief: I don’t care what it was.  Find the damn thing!

Every one knows dolphins aren’t fish!  In real life, that’s about as stupid as someone saying this:

FBI Chief: So, you’re sayin’ a little toddler started the car and drove off with the briefcase!?

Scientist: Actually, it was a midget, sir.  They are very smart…

FBI Chief: I don’t care who it was.  Find the damn thing!