This comes from a discussion I had with a friend on email - there is no trick to this, it is simply a bit of school physics, but I was surprised how much discussion it gave rise to. I thought I'd trot it out here - have a think about this...
A man who is working from home sets up an experiment with his children one afternoon, as part of their home-schooling. They like that kind of thing, apparently.
They place a boat in their (very small) swimming pool. The man climbs in, and takes on board a number of very large stones borrowed from the garden. When the ripples have stopped, his kids mark (very accurately) the water level on the side of the pool (not on the boat, on the pool side).
Once they have done this, the man very carefully drops all the stones over the side into the water. Again, when the ripples stop, the kids mark the water level on the pool side. We may assume that there is no loss of water through splashes, overflow, drainage, leakage or evaporation during the experiment, and that the kids can mark the level with unlimited precision.
OK then - when he dumps the stones overboard, does the water level in the pool
(3) stay the same
No prizes, obviously, just a bit of (supposed) fun. I won't publish any comments for a day or two, so as not to spoil the puzzle for anyone who cares - this will also allow me a couple of days before I have to reveal that there were no responses at all.