The River Locks

A river had seven locks, each controlled by a lockkeeper.

The lockkeepers had a rule: don't open your lock until the one below you signals clear.

One day, the fourth lockkeeper took ill and couldn't signal. The third lockkeeper waited. The second waited. The first waited.

By noon, forty boats were stacked at the river's mouth.

A passing merchant asked why no one had simply walked down to check on the fourth lock.

"That's not the protocol," they said.