First, you will need a gallon of milk that's about a quarter full. It's best if you empty out the contents, but make sure to save it though because you will be needing it later.
Notice on the gallon jug, there is a long straight line from top to bottom. Using a cutting tool, cut along that line till you get about half way down the gallon.
Next put the milk back inside the gallon, put the cap back on, which should help keep everything in place. At this point, the gallon should look like any old gallon of milk to the naked eye.
Leave the gallon of milk anywhere you know your victims will get it, i.e. the fridge. Duh.
When they remove the cap, the opening will become lose and when the gallon is tilted when pouring the milk, all the weight of the milk will come spilling out through the corner of the gallon making a big mess!
This prank is harmless, yet so annoying, since nobody likes to clean up spilled liquids on the floor, especially not milk, which tend to be sticky. Try it out its fun, especially on April Fool's Day, but any day can be April Fools Day when you execute this prank!