The drinking bird?

Question

Today in my physics class, we did a drinking bird lab. I finished all the questions for the lab but one of the questions was

1. What force(s) (and direction of force(s)) is/are responsible for this observed process. [hint: more than one force]

I think it was the force of gravity and weight? but I am not sure. If anyone can give me more details, it would help. Thanks! Please leave serious answers :]

Answer 1

The drinking bird works because of gravity and shifting weight, but those seem to be the same force to me. The second force to me is the shifting of the fluid inside due to the cooling by evaporation and the reheating from room temp. This would be vapor pressure.