The Science Butlers

The Science of non-drip can

If you have a container of water with only one hole and that hole is quite small, then air pressure can stop the water getting out.

You can see with the Red Bull can in the video, when the can is upside down the drink only just trickles out. If any more liquid came out, then it wouldn't be possible for any air to get back in again through the small hole. That would mean that the pressure inside would be less than the pressure outside and so the liquid would be "sucked" straight back in again!

This doesn't work for large holes as the liquid can get out and the air in at the same time. So, for the can in the video, you have to turn the "ring pull" around to make the hole smaller.

Also you can still pour through the hole as then liquid gets out of the bottom of the hole, and air comes in at the top, keeping the pressure balanced inside and out.