system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When I tap a concave glass beer bottle bottom on top of another glass beer bottle it foams up uncontrollably, but if I just tap something flat on top of the other beer bottle it doesnt?
gpt
It's not the shape of the bottle that causes it to overflow, it's the hardness of it.  A beer bottle hitting a beer bottle creates a pressure wave that goes through the bottle causing it to bubble.  If you hit it with something flat but soft like a book it would cause a less severe wave and be less likely to foam (unless you used more force).  Basically anything that causes the bottle to make a clinking noise will have the same result if you apply equal force.