Lambda bills when called; RDS bills while it exists.

A function is charged by invocations and compute time, so a quiet night costs nothing. A database instance is a machine that keeps running and billing around the clock until you stop or delete it. When you hunt for surprise charges, the always-on parts are the first suspects: databases, NAT gateways, load balancers, forgotten instances.

Read it once, then a question tests whether the trick stuck.