Why Do Things Float?

"Hurumph," said the frog. "What a question. The thing doesn't even know what makes it float." He spoke softly, as if he were talking to himself. "Well, see here," he said abruptly. "How about you sit there and I'll see if I can explain, in a manner that will aid your understanding, as I am not used to speaking to creatures that are not as smart as I am, you know." The frog slowly turned to face Henry more directly and regathered his legs under him, then squatted again.

"Well, thing, it's like this," he began.

"My name's Henry," Henry said quickly.

"Okay, Henry, it's like this," began the frog again. "Now please don't interrupt again." And after a short pause, he closed his eyes and began to speak in a slow, studied manner. "When an object sits on the water, the water underneath pushes up against the object. This is called upthrust. If this force matches the weight of the object, then the object floats." He was silent again, still with his eyes closed.

Henry did nothing. He waited. The frog waited, too, evidently, and before long he opened one eye very slowly and peaked at Henry. Then both eyes opened. "Well, aren't you going to say anything? It's polite to thank me, you know."

"Oh, thank you, thank you," said Henry quickly. "Uh, but I still don't know what makes things float, sir frog. I'm sorry. I just don't understand."

The frog breathed a deep, croaky sigh and mumbled under his breath. Then he began again. "Okay, it's like this. When an object is on the water, and the weight is the same as the weight of the water it has displaced," and the frog paused, seeing the look of confusion of Henry's face. "Uh, that means it took the place of where the water was before. Get it?"

"Oh, sure, yeah," Henry spluttered.

"Good. Now, sometimes things are denser than the water, so they sink. Density is measured by comparing the amount of matter in an object and its volume. The more matter is squeezed into an object of given size, the more dense it is. Get it?"

Henry looked puzzled.

"Let's see. You remember that white thing you threw in the water yesterday?"

Henry thought for a minute. "You mean the tennis ball? Yeah, sure."

"Well, that thing is very light. It probably isn't solid, so it's not very dense and in fact its density must be

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