Q. What do you get when you cross a snowman with a vampire?

A. Frostbite.

Explanation:  This is a common type of joke that is funny because you cross (combine) one thing with another (usually two things that do not go together).  In this case a snowman (like Frosty the Snowman?) with a vampire (that bites people).  Funny!  Click this link to learn more about Frostbite.  Frost is the ice that is formed on the ground when the temperature goes below freezing.