I have decks for:

• English vocabulary. I've learned many new words and sometimes get an explanation for a word I had only inferred the meaning of from the context - and guessed wrongly.

• Family facts, mostly birthdays. It's a minor thing really, but I used to not know how old everyone is. And more than once I felt bad when someone asked about the age of a parent and I had to say 'no idea'.

• Random facts I've looked up several times before or that I don't want to have to ever admit not knowing. Like the age of the solar system, the first few digits of Euler's number, approximately when Newton lived. This kind of thing.

What I wish I had a deck for: Math. I really enjoyed doing math and am sad that I'll forget most of the definitions and theorems now that I don't use them regularly anymore. I've tried converting my lecture notes into flashcards, but it's a lot of work that I'm not motivated enough to do.