This is based on a famous thought experiment in quantum mechanics. If you want to know what it means, there is a good write up on Wikipedia. If you don't understand it, don't worry - I didn't either 8-(. I read about Einstein's theory of general relativity when I was younger, and did not understand that either. Lately I've made several attempts on Stephen Hawking's "A Brief History of Time" with the same result.
This model is not a free download, but it was well worth the small cost. It is Schrodinger's famous cat hiding in a rubbish bin (being very unwilling to partake of an experiment with about a 50% chance of a dead-cat result!). The accuracy of the parts is excellent and it went together very well. The gears and cam are beautifully done. You can get it from http://www.flying-pig.co.uk
This one is quite simple to build. The design expects you to print double-sided on an ink-jet (unless of course you have a colour laser at home!). However, the solid blocks of colour on the reverse sides use a lot of ink, so you might like to just leave them white, or colour the reverse sides of the cut-out pieces some other way..
I used 200 grammes per square metre paper-board. No problem at all in my HP PSC1315 printer. 200gsm is about 0.65 ounces per square foot, best as I can work out. 80gsm is normal photocopy paper here in Australia.
I made this for my dear wife, who loves cats of all sorts. We have two real live ones and a vast collection of figurines of various sorts. Here's a picture of the model. It needed a small coin stuck to the inside of the bin lid to make it snap down quickly.
This page http://tardus.net/schrodingersCat.html published: 12 Feb 2007