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<正>夏天是游泳的好季节。就算不会游泳,只要套上游泳圈,照样能在水中畅游。为什么套了游泳圈就不会沉到水里呢?Boats, ships, kayaks, and rafts all have one thing in common—they are used for transport onseas and rivers, because they float.It’s easy to figure out that if they are made out of wood they will float, because wood floatsin water. However, when you see a huge cargo ship or a cruise-liner that looks like a floating
