This Instructable tells you how to make your own air-horn from common house-hold scraps.
With a little practice, each air-horn takes under five minutes to make, so an evening's work can produce enough air-horns for even the largest family to enjoy around the Christmas table, or to dole out as party-favours so that the dear little ones can take some of the party fun home to the parents who didn't help arrange the party... ;-)
Step 1: Equipment and Materials
A 35mm film cannister or similar plastic pot.
(You'll probably have to buy the balloons and straws, but you can scrounge film cannisters from your local photo-developing store. I get 20 or 30 a time from our local "Boots".)