For many minutes neither had spoken.
"Look!" said Tilda, pointing to a distant ripple drawn straight across the surface.

"There goes a rat, and I've won!" The boy said-- "A boat takes up room in the water, doesn't it ?" "0' course it does.

But what's that got to do with rats ?" "Nothing.

I was thinking of Sam's puzzle, and I've guessed it.

A boat going downwards through a lock would want a lock full, all but the water it pushes out from the room it takes up.