glue?

In Reading Class, Stephen was learning about friendship from the book of Proverbs.   One of the fill in the blank questions was this:

There is a friend that sticks closer than _______________________.

Stephen had put the word "yes" in the blank, which of course didn't make sense.  

"There is a friend that sticks closer than yes."  We laughed together and then I asked him what the correct answer was.   

"A friend sticks closer than what, Stephen?"

To which he replied, "Glue?"