Pencil and paper only: can you multiply 2573 by 389? No calculators. And you have to get the right answer, of course. This is not a trick.
“Of course I can,” you reply. “That’s a skill one never forgets!”
But don’t be so sure.
This question came in a post by famous math educator Ben Orlin. You should definitely read that post, in which Orlin explains the origin of the exercise in a test for British 11-year-olds, the subsequent shock of finding that many well-qualified math teachers couldn’t get the right answer (of course they knew how to get it, they just made mistakes along the way), some possibly explanatory context, and gloating responses by many readers in the comments section. Note again that there’s no trick: it’s simply very difficult to get the answer without making mistakes along the way.
By the way, if you (like me) learned the traditional method of “long multiplication,” you may want to try the lattice method. Here is a mysterious example using a different problem: