A turnstile is a rotating gate with arms that spin to let people through one at a time. You've probably seen turnstiles at subway stations, amusement parks, or sports stadiums. They work like this: you insert a ticket or payment, then push through the rotating arms that click as they turn, blocking the path behind you so the next person can't go through without paying.

Turnstiles solve a practical problem. Imagine trying to count thousands of people entering a baseball stadium while also making sure everyone has paid. A turnstile does both jobs automatically: it counts each person who passes through (you might hear it clicking) and physically prevents anyone from entering without a valid ticket or token.

Some turnstiles are full-height barriers that look like rotating cages, while others have waist-high arms. Modern turnstiles often use electronic cards instead of coins or tickets, but they still serve the same purpose: controlling access while keeping an accurate count of who enters.