How to design a deliberately biased coin? For demonstrating basic probability concepts, it would be nice to have a coin-like object that lands heads/tails not in 50/50% ratio, but biased in a way that can be revealed in a short experiment.  What I'd like is to make an object satisfying:


*

*Thin disk shape, say thickness around 1/10 to 1/20 of diameter.

*Lands heads/tails with some given lopsided ratio such as 60/40.

*Feels evenly balanced to the students' hands.

*Can be made by the average machinist, hobbyist woodworker, or 3D printing designer.

*Size not important, but maybe 10-20cm diameter.  It's meant to be a theatrical prop visible to a small audience, not an actual coin.


Would a disk with an interior hollow zone closer to one side than the other do the job? I doubt it, since the coin will still rotate uniformly in the air, exposing both sides equally to any direction, including the floor.   Making both sides "heads" is too obvious a cheat, and I don't want 100/0% probabilities anyway.
Note I'm not asking for any practical how-to workshop details, just the physical principle for designing such an object.
 A: I would suggest making a magnetic coin. Then you can alter its results by having an electric magnet behind the table you are flipping the coin on it.
However, if the magnetic field is two strong it would be easily distinguished by bare eyes. So you have to balance its strength, by flipping the coin numerous times(e.g. few thousands) and calibrating the magnet with the probability of heads/tails.
This way you will not only have an unfair coin, but a variable unfair coin which can be used for many demonstrations.
Anyway, note that even a fair coin has a lot of uncertainty, which means by flipping a fair coin a hundred times; getting 40/60 is not highly unprobable.
A: Maybe this answer won't be useful to you at all, but still. 
Since you mentioned that all this is for a demostration, you might as well use a loaded dice than put in all the effort to make an unbiased coin. Even if the audience is completely new to concepts of probability, a dice will keep concepts as simple as in the case of a coin.
If you are interested in the mechanics, then you can read 
http://en.wikipedia.org/wiki/Dice#Loaded_dice
If it is required that you have to construct it yourself then "shaved die" will be the best option: get a wooden or metallic cube made and file one side to make it asymmetric. See http://en.wikipedia.org/wiki/Dice#Shaved_dice
You will have to see for yourself how much filing goes unnoticed. In any case it will need a few hundreds of throws atleast to reveal its bias.
