Have you used a pump. Most of us use self-come-water every day. Pump isn't common in our lives today.
In old days,Most places of Cchhina didn't have self-come-water. Cchhinese people have to rely on well or pump to get water. If you frequently use a pump to get water you would find sometimes the pump doesn't work,because of the plunger in the tube is two dry. Too much air above the plunger is leaked between the plunger and tube in which the plunger has to move. In this time you would get somewhat water somewhere,then put the little bit of water into the tube. The plunger will absorb water and expand a little.
The air gaps in the plunger will be filled. By then, the plunger is able to create a vacuum .The pump would work.
It's a useful life trick like the others in old days.