For me, an example would be this pretty well known question:
Simplify the following: (x-a)(x-b)(x-c)…(x-z)
If you haven’t seen this question, I highly encourage you to struggle with it by yourself until you find the answer. Whatever you do, do not get any help: you will figure it out. And I promise you it’ll bring a huge smile to your face once you do.
I’d love to hear your personal favorite problems!