have to try this
you can make a cheap homemade bottle valve from a cork and the valve from a old bicycle inner tube.
bike shop gave me a handful of old ones when I asked (the longer thinner type are better)
if you put a bit of vodka (rubbing alcohol works even better) pressurise the bottle a little bit and then pull the cork out you can make a cloud in a bottle
it gives a strong enough seal to make a cheap water rocket aswell (idea nicked from bang goes the theory)