Q: What do you get when you combine a porcupine and a turtle?
A: A slowpoke!

Explanation: A porcupine is an animal that has needles which are called quills.  The quills are sharp on the end and will hurt if they touch you, if they poke you (like when you get a shot in the arm).

Turtles are animals that usually move slowly.

This joke is funny because it combines the idea of a slow turtle and a porcupine with quills that can poke you into a slowpoke: a slowpoke is someone who does something too slowly.

Here is another animal that moves slowly: the sloth.