A raindrop enters Lake Superior. It leaves 190 years later.

Drop a raindrop into Lake Superior. How long before that water reaches Lake Erie? Days? Months? Guess.

Here is the route — and here is the trick. Every river on it, end to end, takes only about three days.

The delay is the lakes. Water mixes into a volume that drains slowly, and the average wait is volume divided by outflow. Lake Superior alone: a hundred and seventy years. Lake Huron adds nineteen more.

Total: about one hundred and ninety years. Two centuries, for one raindrop. Computed from a public river dataset — the full calculation is on this channel.