A horrible surprise. In order to concoct, you need a metal thermos, and a car. Place dairy products inside the thermos. Milk, cottage cheese, heavy cream, shredded cheese. Anything that goes bad with haste. Seal the thermos and place it in the back window of your car. A good thermos is airtight. Allow it to sit all through the summer, even multiple summers if you're patient. When the time is right, open it and unleash the vile stench onto the world, be it throwing it ON somebody, or into some jackass's convertible in a hot parking lot.