Mold is a moisture problem, not a cleaning problem.
The dark spot under the bathroom caulk
It usually starts small. A faint discoloration along the shower caulk, a musty smell in a closet that shares a wall with a bathroom, a patch of soft-looking drywall behind the toilet. The instinct is to reach for bleach and scrub until the spot disappears. That can work on a hard surface, but it addresses the symptom and not the cause. If the moisture source is still there, the mold will come back, and each time it does, it may be established deeper into materials that are harder to clean or may need to be removed entirely.
The moisture comes first
The EPA's position on household mold is direct: the key to mold control is moisture control. Fix the leak. Dry wet materials within 24 to 48 hours when possible. Keep indoor humidity below 60 percent, ideally between 30 and 50 percent. A simple humidity gauge, available for a few dollars, tells you where your home stands. Without that baseline, you are guessing about conditions that mold is not guessing about at all.
There is a practical difference between a small mold patch on a tile surface and a larger problem involving drywall, carpet, or hidden spaces behind walls. EPA guidance suggests that a mold area under roughly 10 square feet can often be handled as a household cleanup, while larger areas or situations involving significant water damage call for more extensive remediation guidance or professional evaluation. That boundary is approximate, not absolute, and other factors matter: whether the water source was contaminated, whether vulnerable household members are present, and whether you can actually reach and correct the moisture source.
Scrubbing visible mold without fixing the water source is cleaning work that will need doing again.
Look for water, not mold
The action step asks you to walk through your home and check three moisture-prone locations: under a sink, around a window, and in the bathroom. You are looking for active leaks, condensation, visible mold, and any evidence that water is reaching a place it should not. The critical judgment call in each case is whether what you are seeing is a cleaning problem or a moisture-source problem. A cleaning problem is surface mold on a tile that gets scrubbed and stays gone because the bathroom fan works and the caulk is intact. A moisture-source problem is mold that keeps returning because a pipe is leaking behind the wall, and no amount of surface cleaning will resolve it until the pipe is fixed.
The reading for this module
This is the core reading for the module. Take it with your earlier work beside you.
Check three moisture-prone locations.
Walk through your home and check three moisture-prone locations: under a sink, around a window, and in the bathroom. Note current humidity if you have a gauge, any visible mold, any active leaks, and any condensation. Identify whether each finding is a cleaning problem or a moisture-source problem.