Cutting through the problem like a hot knife through butter was Brit-blogger Ed. He got everything right except for his last statement which was a little off track (and actually contrary to the first two). Despite being picked up by both Sarah and Eric I completely failed to notice this and initially gave Ed first prize.

So in a rather embarassing (to me) re-awarding ceremony, the first prize is actually going to have to be shared between both Ed who was first through the door and got the logic right and Eric who got both the logic and the fact that bricks are denser than water.

Well done both of you, you certainly did a lot better than I did in my interview. If I recall correctly my response was:

1. sweat nervously
2. answer: stays the same
3. answer: goes up
4. find door

If the answer doesn't make any sense to you then here's a bit more of an explanation. It may or may not help:

Pressure in water decreases as you go downward. This is because the water at the bottom has to support the water above it.

Because the pressure increases with depth, the pressure at the bottom of an object in water is slightly greater than the pressure at the top. This means that there's a net force pushing the object up - buoyancy. Do the figures and it turns out that the strength of this force is equal to the weight of the displaced water.

So, the water is pushing the object up (buoyancy) and gravity is pulling the object down (weight).

If the object's weight is greater than its buoyancy it sinks, if it's the same it just hangs in the water like a jellyfish and if it's less then it floats.

The buoyancy of the brick is equal to the weight of a brick sized lump of water and since a brick sized lump of water weighs less than a brick-sized brick, the brick sinks.

When the brick is in the boat it pushes the boat down. As the boat takes the weight of the brick it sinks into the lake and displaces enough water to provide the extra buoyancy required to keep them both above water. If it didn't do this they'd both sink - try putting a brick in a paper-boat.

However, when the feller chucks the brick out of the boat, the boat bobs back up again while the brick now only displaces its own volume. Since a brick-volume of water is less than a brick-weight of water, the level of the water in the lake goes down a little wrt the shore.

(Jeff gets second place prize for making me laugh)