Grapes in the sun

This morning 100kg of grapes were put in the sun. They were 99% water, but through the day they lost moisture to evaporation and now they are only 98% water. How much do they weigh now?

Gut answer: something close to 100kg.

Correct answer: 50kg

Comment: This is a wonderful puzzle because it is so counter-intuitive.

When the grapes are 99 per cent water, the ratio of non-water to water is 1 to 99. When the grapes are 98 per cent water, the ratio of non-water to water is 2 to 98, or 1 in 49. Since, after the evaporation, the non-water is unchanged at 1kg, the total weight must be 50kg.