I think it goes down since when on the boat the brick is displacing the brick's weight in mass of water.

When in the water it only displaces its volume, which (especially since bricks are porous) is less.

Ie the level of the lake goes down since bricks are less dense than water.

Though I'm probably wrong.

This post has been removed by a blog administrator.

I deleted my last post because I didn't explain it very well.

My guess is that the water rises, based on my understanding of the concepts of buoyancy and displacement.

Essentially, the displacement of the lake's water by the boat (and its matter) is very slight due to the overall buoyancy. But since a brick's buoyancy is virtually nil, the lake level must rise due to displacement. However, throwing the brick overboard does not affect the boat's buoyancy in any meaningful way.

So in essence, the water displacement caused by the brick being on the boat does not equal the displacement when the brick is in the water.

I could be totally wrong, but it makes sense to me.

Well, since the other two options have been taken, I'm going to say that the water level stays the same. I don't really believe that (I agree w/ Ed), but if they both happen to be wrong, I want the award :)

Are bricks less dense than water?

I know water is heavy, but I am imagining a block of water the size of a brick and it doesn't seem as heavy as the brick.

My guess is the water rises slightly, using the same logic atacrawl uses. The brick's weight spread across the boat's hull is displacing much less water than it's volume would.

This reminds me of the puzzle that involves the passenger on a moving train jumping straight up in the air. Do they land in the same spot, or further back in the train?

Well I think the level of the water goes down, but my logic is a little different than Ed's. First of all, I assume that the brick is more dense than the water, otherwise it would float. However the brick+yacht+person is less dense than the water and it does float.

Now imagine that instead of a brick it were a ping pong ball sized chunk of a heavy metal that happened to weigh as much as the brick. When you do the thought experiment with this object in place of the brick, at the end the water would be at a lower level than the version with the brick since the ping-pong ball sized chunk displaces less water. Conclusion from thought experiment: the more dense the item, the lower the resulting water.

Now the water would remain at the same level if the brick matched the density of the brick+yacht+person, but we've already established that it is more dense.

Therefore the water goes down.

Eric