A small interruption can dissolve understanding that took an hour to gather.

Deep attention is expensive to build and cheap to destroy. An hour of careful reading assembles a structure in your head that one question can knock over, and returning is not a matter of picking up where you left off. That asymmetry is what makes interruptions so costly. Rebuilding takes far longer than the interruption itself did.

Protect the hours where something has to be assembled rather than merely processed. Say when you are in one, and be careful with other people’s. If you must interrupt somebody, ask whether now is a good moment before opening with the question.