Ship of Fools
Mr and Mrs Strange row out to the middle of a large lake, carrying a rock in the bottom of their boat. Then they throw the rock overboard (don't ask why - they're just strange), and it sinks to the bottom of the lake.
Does the water level in the lake rise or fall (relative to the shore)?
If you are stumped by last issue's puzzle, here is the solution.
For some challenging mathematical puzzles, see the NRICH puzzles from this month or last month.