A boat, a lake, a handful of steel marbles. The question seems like a physics one, but the answer depends on something more subtle than simple weight. It is worth thinking about it slowly.
The boat and the marbles in the lake
Riddle statement
You are in a boat in the middle of a lake. Inside the boat is a box full of very heavy steel marbles.
If you throw all the marbles into the water, does the level of the lake rise, fall, or stay the same?