Caulk seals the gaps where air and water sneak in — around windows, tubs, trim, and siding. It's a cheap material and a quick skill that pays off in comfort, lower bills, and prevented water damage.
Pick the right caulk
Use a flexible, paintable acrylic-latex caulk for interior trim and gaps you'll paint, and a waterproof silicone or kitchen-and-bath caulk for tubs, showers, and sinks. Exterior gaps need a weatherproof, paintable exterior caulk.
Prep the surface
Remove old, cracked caulk and clean the gap so the new bead bonds. Caulk over dirty or loose old caulk just fails again. For tubs, dry the area completely first.
Apply a clean bead
Cut the tube tip small at an angle, apply steady pressure, and run a smooth continuous bead. Tool it with a wet finger or caulk tool to press it in and smooth it. Wipe excess before it skins over.