Two drunks are arguing about how much water is in a barrel. Its a normal barrel (a cylinder) They have a bet on. One thinks its more than half full, the other thinks its less than half full. They have no measuring devices (and they're drunk so probably wouldn't be able to use them lol), so how can they resolve their wager?
If you put the barrel in water then it should sink no further than the internal water level and you can see whether it's more or less than half full.
Alternatively they could drill a hole exactly half way up the barrel and if water comes out it's more than half full.
One of the drunks could get into the barrel and out again. The line at which his clothing is wet will give the water level in the barrel when he was in it. This will only work if the one who is betting that it is less than half full is too drunk to realise that the water level will rise when someone gets into it.
If they carefully tip the barrel until the liquid just reaches the top:
Then if they can see the bottom of the barrel then the barrel is less than half full (left hand side in picture), and if they can't see the bottom then it is more than half full (right hand side in picture).