Asklemmy

A completely random ordering of a deck of cards. You can have a deck pre-stacked in this order, learn some false shuffles, have someone pick a card and place it back anywhere they want without marking its location in any way, and when you inspect the deck you know exactly what their card is. And they'll never guess that the way you did it was memorizing the order of every card in the deck.

I'm sure there are a lot more advanced ways to take advantage of this, just a handy ability to have in your back pocket (literally).

Do you remember the Fibonacci sequence? You can use it to convert miles to kilometers .

2 mi ~= 3km

5mi ~= 8km

8mi ~= 13km

13mi ~= 21km

And so on.

Learn some alphabets of foreign languages. Russian is fun because some of the characters looks like English letters but have completely different sounds. Korean is also cool because it looks crazy complex but it's actually extremely simple.

For day-to-day purposes, if you are used to Fahrenheit but not Celsius or vice versa, and all you want to do is get a rough sense of how warm or cold it is outside without having to do arithmetic involving fractions in your head, then remember that there are two temperatures in Celsius that are roughly the same in Fahrenheit but with their digits transposed: 16° C ~ 61° F, and 28° C ~ 82° F. You can then roughly interpolate/extrapolate by about 2° F for every 1° C.

Simple recipe formulas that are scalable