[Seany (OP)]

Make your own bicycle lock.
It can be lighter, cheaper, and much more fun than any lock you can buy.

Here's one made of cardboard!
It's perfect for use in Japan and other nice places where people don't like to steal.

[Make it Lady]

They obviously don't have scissors in Japan!

[Karen W.]

There seems to be no need for scissors either!! Lol...lol...

[Bored chemist]

Also, unlike an ordinary lock, it won't mess up your bike if you forget about it and ride off without unlocking it.

[RD]

Very chindogu

[Nobody's Confidant]

Once I made an ICBM out of cardboard, mustard, and a couple pianos. The CIA doesn't let me give out specs though..[]
So! I invented a railway spike rifle using a crutch, pressure cooker, steam gauge assembly, and fission battery! (Last one may be a tad hard to find.) It shoots railway spikes! How cool is that?!
Trainyards aren't to happy about that though....
So I invented a dart gun using a toy car, surgical tubing, paint gun, and a scorpion poison gland. (Last one is a little difficult to aquire). Sold it to some international poachers with a tracking chip in it. The CIA picked them up later. It made up for the whole ICBM thing.

[Make it Lady]

chocolate fireguard, that's chindogu. The above all sound very useful to me. My brother made a handgranade out of shaved off match ends. He blew his eyebrows off!