Multiplication tables. Yeah, I know, but not quite how you think!
Up until middle school, I got away with calculating bits of it really fast compared to others who just knew it after trying to recall for four or five seconds. So teachers assumed I’d learned them, too. Then, suddenly, they all either had it from rote memory, or didn’t have a clue. I was still clumsily calculating every time. I somehow didn’t get how others had learnt it by rote, like they were somehow working it out faster than I was. I spent time trying to work it out even faster!
Then one maths teacher got annoyed that half the class didn’t know all their times tables (we were the top class in our year), and made us all stand up and answer questions rapidly or not sit down. He’d go round twice. Remaining standing at the end was embarrassing. Then I realised, after a couple of months, I knew 7x6 (his favourite) without calculating it and wow! I got it, suddenly.
Years later, my friends on my computer science degree knew I was “good at math” - and I was, especially algebra, and simpler math calculations - the latter from all this practice! Even now, years later again, I still haven’t rote learned some of the 8x table. 8x7 is 8x8 - 8, and 8x6 is 8x5 (=8 x10 /2) +8 in my head. But 7x6? 42, like a reflex!
If you have kids, make sure they can calculate and know those tables reflexively!
Oh, and shoelaces? If you have 3 minutes, this might solve the problem: