[secondForty]

Neat example.

So nobody in the class just waits until you're not looking and weighs the whole bag with the scale?

[bittercynic]

You could make that strategy difficult by using a big, floppy bag and a scale with a small platform.

Or simply a scale incapable of measuring a weight much larger than the heaviest piece of candy.

[mturmon]

Making the bag itself heavy would also help.