Squash.
A human.
Because dogs can't whistle. (X-post from r/dadjokes)
Rock is playing three chords for a thousand people. Jazz is playing a thousand chords for three people.
Two flutists playing in unison.
Only one side was game
Games.
A monster on roller-skates.
A monster with an all-over perm.
A flat.
You can't zucchini bugs! A family-friendly take on the age old "jam VS jelly" joke.