2digit.org

Blackjack: a common casino game where you try to reach 21 points (without going over).

Catch 22 - This comes from the novel of the same name written by Joseph Heller in 1961 and is a paradoxical situation which you cannot escape due to contradictory rules.

.22 Long Rifle rounds: despite the name, this is a pistol caliber and generally considered the weakest caliber you can buy. It's mostly used for target shooting.

Number of pairs of chromosomes in human DNA, from which the company 23andMe derives its name.

The "27 Club": A group of famous musicians who died at 27 for various reasons, leading to a superstition that it's a particularly dangerous age.