Although it may not look like it, Tom running with rocks is a favor for Huck.

Why?

Because when Tom throws the rocks in a mud puddle, Huck likes to bark at it.

And then Tom could (make his mom) gather (A LOT) more rocks.

And Tom could throw them in his pool so Huck could bark at them.

And Huck and Tom would both think that was the most fun EVER in the world.

And mom would be super happy she thought of such a fun and educational (?) game.