What boy doesn't love an innocent prank? I know mine do! I remember my brother having one of these (and I may have too) when we were kids. They are SO easy but are a huge hit with the Y chromosomes (the X's as well). Let's see how they're made!
First get a ketchup and/or mustard squirty bottle (I got the pack of 2 for $1.50) some red and yellow string, and 2 small washers or some sort of light item to weight the string down a bit. (Nylon string is best for this but I couldn't find it thick enough for a reasonable enough price)
Tie the small washer to one end of the string
Put the string through the lid of the bottle from the bottom up
Tie a knot in the top of the string so the length of the string matches the length of the bottle where the washer is slightly resting on the bottom of the bottle. (The washer is used to weight the string so that it slides back in the bottle easily, without this it is hard to get the string back in the bottle easily)
Put the lid back on the bottle and try it out!
Then go prank someone! If you can wait it's much more fun when having ketchup out is believeable, however, when you're 6 and 8 it's just too hard to wait!