Step 1: The idea + materials
how does it work ?
well it's quite simple, when transferring the air (or water) pushes the ball up (thats why we use a really light ball), passes around it and gets out the other end. when air (or water) tries to go the other way the ball is pressed to the bottle creating a seal.
notice : the air (or water) flow can move only in one direction - upwards, and the system works only when held in an up-right position with the O-ring at the bottom side ( you can put O-rings on both sides so it won't matter which is bottom ) !
so, to make this you will need these materials :
1. 2x plastic bottles ( without any paterns near the cap )
2. 1x table tennis ball
3. cutting tool
5. epoxy glue ( super or crazy glue arn't meant to seal stuff )
6. optinal : for maximum sealing use an O-ring (*)
(*) to know what diameter to use - after cutting the bottle (accotding to the next steps) put the ball in the top side of the bottle and see where the ball toutches the bottle, then measure the diameter of the bottle at that line (asumin that the inner diameter is clise to the outer diameter) and that is the diameter you need - it doesn't have to be exact becaause the O-ring itself had defferent inner and outer diameters.