Gravity is simple: whatever you shake or wipe from higher surfaces falls down. If you sweep or mop the floor first and then dust shelves and fans, all that dust settles right back on your clean floor. Result? You end up cleaning the lower parts twice.
Dusting top-to-bottom—ceiling fans, high shelves, wall frames, then tables, then chairs, and finally the floor—means everything that falls gets caught in the last step. One flow, one direction, no going back.
It’s a small change in order, but it saves energy and gives a more thorough result.
