Caulk
Flexible sealant that closes gaps to keep water and air out.
Caulk is the bead of flexible sealant around tubs, sinks, counters, windows, and exterior trim. Its job is to keep water and air from getting where they shouldn't. Caulk dries out, cracks, and pulls away over time — and a failed bead lets water seep behind surfaces, causing rot and mold. Inspect it yearly and re-caulk gaps before water finds them.