heat to make the water evaporate, a hole with gum to patch it when the water gets down to the proper weight. A straw to sip up the excess liquid.

its a real simple answer my friend. although that would work.

the answer is just add holes

Simple, yet not so simple.

How much does the bucket weigh?

The bucket and the water together weigh 20 pounds. The only way you could just randomly make a hole anywhere in the bottom of the bucket is if you have a 12 pound bucket and you let all the water drain out.

Otherwise:

Water weighs about 8 1/2 pounds per gallon. To make it weigh 12 pounds, you would have to figure out how much volume a little less than a gallon occupies in the TOP of the bucket (unless the sides are straight, it's a modified cylinder or a truncated cone--so you'd have to do some math) and then place the hole accordingly to drain off the proper amount of water.