Why do DNS changes take time to show up (propagation and TTL)?

When you change a DNS record, the new value does not reach everyone at once - it spreads as the old answer expires in caches around the world. That spread is called propagation, and how long it takes is set by the record’s TTL:

• TTL - short for time to live; it is the number of seconds each record may be cached before a resolver has to ask again for a fresh answer.
• Why old answers linger - until a cached record’s TTL runs out, that resolver keeps serving the old value, so some visitors see the change before others.
• Planning a change - lower the TTL a day before a planned move so caches expire quickly, then raise it again once the new value is stable.

A change is not broken just because it is not live everywhere yet - give it up to the TTL before assuming something is wrong.