A favorite college prank from the mid 1960s was to fill a condom to the size of a watermelon - probably close to 5 gallons of water. It takes 2 people to carry it on a towel - very carefully. Then roll it very carefully from the towel onto someones bed. Cover it with a sheet so they don't know exactly what it is at first. If they don't know the trick of rolling it onto a towel to move it (often even that didn't work) - and try to pick it up bare handed, it breaks. Leaving them with about 5 gallons of water in their bed.