Have a weighing scale? Have a minute? And do you have a friend that's gullible? Then try out this prank.
All you need:

Weighing scale.
A Family member or a friend.
A minute.

Step 1: Instant Weight Gain/loss!

I think it would be mean to make a bigger person "lose" weight. I don't think you should do that. But anyway, take a normal person and make them 40 or so pounds heavier! Here's how:

Find the click-wheel thing on the side of the scale, thats where the one on mine was, so i don't know if other's have it. Electronic ones probably wont. Move the wheel to make the scale off balance: the second picture below is when noone is on the scale. when someone steps onto it, they seem 40 pounds heavier. Try it! It's hilarious, just don't be mean because weight is a touchy matter for some people.