"Yes, of course. Because the things that float are lighter than the things that sink."
"Not exactly. That log there is too heavy for you to lift, while you can carry a bullet between your thumb and finger. The log is many hundred times heavier than a bullet, but the log will float while the bullet will sink always."
"That's so," said Tom, "and I don't know what does make some things float and other things sink."
"Did you ever set a teacup in the water and see it float?"
"Yes, many a time."
"But if you fill it with water it will sink, won't it?"
"Yes, of course."
"Well, now I can explain the thing to you, I think. If a thing is heavier,—the whole thing I mean, than the amount of water it displaces,—that is, if it is heavier than exactly its own bulk of water, it will sink; but if it is lighter than its own bulk of water it will float."
"Oh, yes, I see."
"Now a bullet weighs a good deal more than its own bulk of water, and so it sinks. A log weighs less than its own bulk of water, and so it floats. An empty teacup weighs less than a solid body of water equal to it in size, and it therefore floats. If you fill it with water, however, you increase its weight without adding anything to the amount of water it displaces,—or rather, as you let water into all the hollow space, you lessen by that much the amount of water it must displace in sinking without taking away anything from its weight, and so it sinks; or, if you break the teacup you lessen the amount of water it must displace without lessening its weight, and so it sinks in that case, too. Do you understand that?"