A jar. With nothing in it. Suspiciously close to you.
This buys you nothing. No XP, no runs, no levels, no rank, no advantage of any kind. Money never buys a better answer anywhere on this site, and a tip does not even buy a run.
What it does pay for is the site staying up and getting bigger: 30,019 problems that stay checked, a formula sheet, a Daily that gets harder through the week, and whatever comes at 1,000 players. You get one thing back: Thank You Green, a name colour that tells everyone on the leaderboard you chipped in.
Minimum $0.50, because Stripe will not process less. Not our rule.
Not into it? Genuinely fine. Go and beat somebody on the leaderboard instead, that is free and it annoys them more.