Ross–Littlewood paradox

The Ross–Littlewood paradox is a strange situation involving a jar filled with marbles. Here is how it works: Every week you take out one marble, and then replace it with two new marbles. After doing this every week for a long time, you'd expect to have way more marbles in the jar than when you started - but somehow, the jar still only has the same amount of marbles in it! It's like magic!