How does it work?

Forced perspective is an easy concept to explain, basically the object you wish to appear large is closer to the camera than the small object. For instance one of the most recent well known examples of forced perspective is in Lord Of The Rings using Gandalf the wizard and Frodo the hobbit.
Because the film makers needed to make Gandalf appear to be much bigger than Frodo scenes with them interacting with one another had to be shot with Gandalf closer to the camera than Frodo.
I'm having trouble finding photos to demonstrate this but if you've seen the film you'll understand.

I've found some footage on Youtube of force perspective:-

http://www.youtube.com/watch?v=8ddVo1c_1Gs

http://www.youtube.com/watch?v=bZYh_HRY70g

The other trick with forced perspective photos is having realistic miniatures available so that you can make someone appear giant by the angle of the shot.

I've done a couple of simple shots myself using my own hand.





I think the apple photo is reasonably effective and the lighting seems to have worked quite well naturally without any adjustments in Photoshop. The kettle photo doesn't quite work because of the surface that the kettle is resting on and the curvature at the base of the object. I think the apple picture may work even better if it is levitated slightly.