If I didn' t know any better, I' d think you were just pretending to be dumb to get people' s sympathy.

Don't play dumb, I know you ate the last piece of cake.

While most of the class seemed to find this hilarious, my teacher was irritated, as if I were being deliberately obtuse.

I know you were only playing possum.

It could have been the "I-don' t-want-to-explain-this, " variety of lie, or the "Ambushed! Play dumb!" variety or it could have been true.