You will need:
a small jar(baby food size)
a bit of fruit(chunk of banana)
Place the fruit in the jar and cover tightly with plastic wrap. With a sharpened pencil poke 3-4 tiny holes in the plastic, just big enough for a fruit fly. Place it on your counter and watch it with morbid fascination as it fills up. Take it outside to empty it out every few days and start over. (the flies can squeeze down through the hole, but can't crawl back through the jagged edges of the holes)
This is my contribution for Kimba's DIY Thursday, so hop on over there to check out what else she has to offer!