Go the local dump or local charity shop, buy a bike, it does not matter too much if it's a kids bike, as long as it's got gears.
Strip the bike down one section at a time and reassemble it, replacing worn stuff as you go along.
Flog the finished result on e-bay for whatever you can get for it.
The lesson here is not how to make a profit (although you may) but how to fix most things on a bike.
I did this aged about 12, it took me most of a summer holiday and with advice (but not help) from the builders of the house next door.
(It helps to have a similar working bike so that you can compare what it's supposed to look side by side)
I've since done roadside repairs of cassettes, chains, wheels, frames and racks with minimal tools, and once, with no warning, to the amazement of Shannon Airport staff stripped down two bikes inside 20 minutes in order to fit them into a box for flying home.
The only thing I've never managed to get right is gear cables, and I can get you home, but not necessarily with all the gears all the time