Because they always leave you stumped!
Cuz she left the plunger in the toilet. - Jackie Martling
If left for 400 years, the yoghurt will develop a culture.
Nevermind, this riddle is too easy.
An Envelope EDIT: My deepest Canadian apologies to those who are calling this a riddle. I always took it as a cheesy joke
It's been nice 'gnawing' you!
The scaffolding
It was stumped.