What does this joke mean, please explain?

Question

I found it on a popsicle:

When is a pillow like a mug of root beer?

When it has a head on it

I don't get it

Answer 1

WOW! Sounds challenging!!

We lay our heads on the pillow.

When you drink a mug of rootbear, it's bottom's up (We raise our heads)

:-)

Answer 2

That's a dumb joke.

But:

Beer has a head on top and so would the pillow. Head when you are talking about beer is the foam stuff on top.

Answer 3

You put your head on the pillow at night, right?  So your pillow has a head on it.  The fizz on the top of a mug (or glass) or root beer, or real beer, is also called a head.