Price $3
You've probably seen someone do this before, but I'm reminding you. Plus, you could easily find new ways of using this in pranks, etc. Simple...go buy a 2-liter of Coke and a pack of Mentos. All you do is carefully & quickly drop the entire pack of mentos, unwrapped, in the bottle and place the cap back on the 2-liter. Do not screw it on more than just a tiny bit. The 2 items will react with each other and cause the 2 liter to basically explode upward. Of course, be sure to stand back.
PS...This will blow the mind of anyone under the age of 10.
Cheap and fun things to do around Minneapolis/St.Paul