A simple science demonstration where water (coloured red) is heated in a conical flask. As the air expands in the flask on the left, it travels in the glass tube to the flask on the right which contains cold water (coloured blue).
Eventually the water boils in the left flask, which causes steam to go into the right flask. The steam bubbles condense and so don't bubble to the top.
When the heat is turned off, the left hand flask cools and the steam molecules condense back to water, making less pressure in the left flask. Air pressure is then able to push the water from the right flask to the left. This happens very suddenly when the cold water reaches it and the steam condenses extremely rapidly causing a rapid transfer of water.
Senior Chemistry students can be heard commenting and exclaiming when the surge occurs.