A sack of potatoes loses 1% of its water and somehow loses half its weight

50kg — half the original weight, from what looks like a tiny 1-point drop in water content. The trick is that the 1kg of non-water 'dry matter' never changes, but it has to go from being 1% of the total to 2% of the total, which means the total weight has to be cut in half. It's a favourite example of how percentages of a shrinking whole can mislead intuition badly.