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Effects with cola cans are popular. Sometimes the cans appear to crush by themselves, and in other effects the cans either refill themselves or re-seal themselves. I'm not going to reveal an effect with this tip, but instead will reveal a 'method' to empty a cola can in preperation for the effect of your choice, and I'll make a suggestion as to how you can use these cans to produce a stunning effect.
Take a cola can and very gently pull back the tab until you hear the typical de-pressurizing noise. Once you hear the sound alerting you that the inner pressure has been released, stop and look at the tab to make sure you have not opened the tab too far. The hole should be barely noticeable. The trick is to now get the cola out of the can. Push the tab back into it's original position; make sure the top of the can is clean; and then begin to suck the cola out of the can through the small opening. There may be other ways to get it out, but this is the simplest.
Once done, you'll have an empty can that for all practical purposes appears to be sealed.
If you want to refill the can, you can submerge the can in a larger container of liquid, weight it down, and allow the can to refill itself over a period of time. You can remove the cola from a cola can and refill it with a clear carbonated drink, and empty a clear liquid drink can and refill it with cola. This will provide you with a good basis for a custom effect.