英語(yǔ)時(shí)差:水上漂

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It's summertime, and you're on vacation. You sit beside a quiet pond, fishing, trying to relax, trying to think about nothing at all. You watch the leaves stirring, the water rippling in the sunshine, and the water bugs standing on the surface of the water.  How do they--Rats! Now your brain's working again, wondering how those bugs do that. Well relax, it's Moment of Science to the rescue. Today we'll explain Standing on Water.

There are two things involved in this trick: one is a property of the water, and the other has to do with the bug's feet. Let's start with the water.

Water molecules naturally stick to each other, due to a strong chemical attraction between the individual molecules. At the pond's surface, the uppermost water molecules experience this attraction as a strong pull toward each other, and downward from the water beneath them. This creates what's called "surface tension"--a somewhat compressed layer of water molecules right at the pond's surface. You can think of this layer as being like a tight film over the surface of the pond.

How does this help the water bug? If the surface water stuck to its feet as well as it stuck to the water below, the bug would be pulled under. As it turns out, the bug's feet are covered with waxy hairs that are "hydrophobic"--which means they chemically resist contact with water. This combination of strong surface tension and water-resistant feet is what lets our bug stand on the water, without even getting wet!

On our next program, we'll learn how water bugs move across this surface. Meanwhile, get back to your fishing!