Q:

Mysterious loss of weight

Now an entry into Fortnightly Topic Challenge #41: Short and Sweet
A man enters a room and presses a button. Within seconds he loses 20 lbs body weight. How did he lose the weight?

A:

Since it's tagged 'lateral-thinking', an alternative that blatantly ignores the question's title is that:

 The man is in the UK, and he just pressed 'OK' button to confirm a £20 purchase

A:

He entered

an elevator and pressed the down button. The downward acceleration from the elevator reduces his apparent weight.

A:

 He presses the button to start a machine. It catastrophically fails within seconds and takes off the lower part of his leg, which weighed 20 lbs.

