“How Many Nails?” Friend, Jan. 1995, 46
Directions: Place a drinking glass on a flat surface, fill it to the brim with water, then put in nails until water flows over the rim. Add each nail carefully, holding it near the point and sliding its head gently beneath the surface of the water before releasing it. Count how many nails you added. Now try the experiment again with a glass that has a wider mouth. How many nails are you able to add this time?
Why So Many?
Why did it take so many nails to make the water overflow? First, the nails do not take up nearly as much room as they appear to, because water fills the spaces between them.
Also, water will bulge well above the tumbler’s rim before it will run over. This is because it has surface tension, which means that the water that has contact with (is touching) the air acts as if it is covered with a thin “skin,” which stretches when you add nails carefully. And because more water is in contact with the air in the wider-mouth glass, you should be able to put more nails into it, even if it holds a little less water to begin with than the first glass.