The Potato Paradox is a really weird mathematical paradox that I found in Alexander Kruel's newsletter.
Here's the description of the paradox from Wikipedia:
Fred brings home 100 kg of potatoes, which (being purely mathematical potatoes) consist of 99% water. He then leaves them outside overnight so that they consist of 98% water. What is their new weight? The surprising answer is 50 kg.
At first, you think it can't be true. Then you do the math and realize it is true. Then you realize that the paradox arises because percentages are weird; and it doesn't feel like a paradox any more.