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.