Q: What did the dog say when he rubbed sandpaper on his tail?
A: Rough, rough!

Explanation:  Another wonderful, classic dog joke!  Sandpaper is rough, it is used to make things smooth.  In English, when a dog barks it says, “Ruff.”  This joke is funny because it plays with two words that sound the same, ruff and rough.

Rough, by the way, can also mean difficult:  I had a rough day.