Magic cans

I saw in the internet a demonstration in a science fair and I have enjoyed repeating the experiment.

This is the result:

It is not a camera (or digital) trick. I am not cheating. Check these close-ups:
There is no trick, simply add certain quantity of water or soda to the empty can. My estimation is around 15 cl but it is not critical. Just try. Drink your soda and try from time to time.

What is the explanation? The person presenting the experiment in the science fair said that the center of mass is over the base but it seems to me that it is like saying that it is raining because you see rain drops. It describes the situation but not the cause. I'd like to find a mathematical model estimating the amount of water and also this intriguing situation:
Apparently it is impossible to do it with a double can (50cl). The base in both cases seems the same at first glance put perhaps there is a silly explanation in this direction. If not, my guess is that the weight of the can (and its distribution) is not negligible. It seems very strange because the water is heavier by far.  Maybe the law of the lever is involved here.

Here there are a couple of photos of the whole family. The amount of water employed in the experiment with the soda cans is in the bottle.

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I have found in the supermarket somewhat special cans: one of 250ml and another of 375ml. Now I am pretty sure that the shape (the height) is very important. The law of the lever is crucial here. I'll try to do the math. I have also considered usual cans with a counterweight. Simply I put duct tape (transparent in the photo) int he bottom of the can and introduced some euro coins.


I had a syringe and I did the experiments using his capacity as unit for the amount of water. Let us say 1u = 6.25ml of water. These are the results:

250ml can	330ml can
Equilibrium interval: 2u - 41u (full can) Notes: with 14u the bottom is covered in the equilibrium position	Equilibrium interval: 6u - 33u Notes: with two euro coins still 33u is the limit with 3 coins one can add 3u more. With 3 coins  we do not need any water and with one coin 1u is enough.

375ml can	330ml can more variations with coins
Equilibrium interval: 11u - 27u (full can) Notes: The height only differs in 1.5cm with respect to the 330ml  can but this makes a big difference in the interval	Equilibrium interval: 9u - 30u putting a coin in the top and another in the bottom. Equilibrium interval: 12u - 30u putting a coin in the top and none in the bottom.