How do you get a UFO to come on Animal Crossing City Folk?

Buy a slingshot at Nook's and then make sure you have it with you every day. After a few days you may spot a flying saucer in the sky! Shoot it down and then search your town to find out where it landed Gulliver will talk to you. Find all the missing space ship parts, return to Gulliver and he'll give you something.