Pick the skill. The engine picks the difficulty.
Every skill carries a practice method taken from the people who documented how it's actually trained — Pólya, Ericsson, Ebbinghaus, Feynman, Miller.
Compute fast and accurately without paper, then sanity-check the answer.
Hold more items in your head at once and act on them without losing the thread.
Hold a target in mind while ignoring everything that competes for it.
See the rule behind a sequence before you see the next term.
Extract the main idea, the evidence, and what the text never actually said.
Follow premises to a conclusion and refuse the ones that only feel right.
Break a claim down to what you can verify, then rebuild it yourself.
Own precise words so your thinking has somewhere to land.
Be understood the first time, by someone who does not already agree with you.