Because they always leave you stumped!
Leave the plunger in the toilet
He left his foot on the clutch.
an Envelope EDIT: My deepest Canadian apologies to those who are calling this a riddle. I always took it as a cheesy joke
Nevermind, this riddle is too easy.
Poul-trees
It needed a root canal.
It was stumped.
OOOOOWWWWLLLL....