What is six inches, goes in your mouth, and it's fun when it vibrates? A toothbrush.
Vibrating Toothbrush
Explanation
Explain Bear
Listen up, buttercup. The joke makes you think of somethin' naughty because of the "six inches, goes in your mouth, and it's fun when it vibrates" part. But BAM! It's just a toothbrush. It's funny 'cause it throws you for a loop. You probably don't brush your teeth, which is why you didn't get it.