Mind

From The Encyclopaedia Expurgatia

A thing that's terribly easy to lose, probably because nobody's sure what one looks like. Because of this whole 'not knowing what one looks like' issue, lost minds are seldom found, and, unlike pens, cannot easily be purchased at the grocery store.

Even if they could, you'd never be able to find just the right one.

Minds can also be wasted, which is usually done alongside the wasting of money through years of education, just so the individual wasting his mind can wave a degree around while discussing the deep, fundamental difference between the mind and the brain, how it makes them superior to animals, and that's why they've just interrupted that obviously pointless thing you were doing just a few moments ago.