ok friends,

it's time to post the solution of my riddle.

to address the solution, first imagine the man in the boat without the stone and sign the level of the water.

1. add the stone into the lake and sign the level 1;

2. add the stone into the boat and sign the level 2.

it is clear that the level 2 is higher than the level 1 because the stone in the boat displaces a volume of water equal to its weight (principle of archimedes), while the stone in the lake displaces only a volume of water equal to its own volume.

this means that the level of water will decrease when the man throws the stone out of the boat.

the difficult here is that this seems a problem of weight but is a problem of volume.

NOTE.

i've read some of your answers using mass instead of weight. because the gravity is the same both for the stone and the water, an equal quantity in mass is also equal in weight so the principle of archimedes is true both with mass or weight.

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now a new one similar to the first, that you can experiment in your kitchen

imagine to pour some water into a recipient but not to reach the edge.

then imagine to throw in it some ice cubes until the water reaches the edge.

(it is not important the number of ice cubes you add, but is important that you reach exactly the edge of the recipient.)

when the ice will be melted, the level of water will increase (going out of the recipient), will decrease or will remain equal?

i must admit, this is a bit more difficult than the first one, but you can try...

regards,

altosax.