Caulk is a waterproof material used to seal cracks and gaps where water might leak through. You'll find caulk around bathtubs, sinks, windows, and anywhere two surfaces meet that need to stay watertight. It comes in a tube and squirts out like thick toothpaste, then hardens into a flexible seal.

Before caulk was invented, sailors used oakum (old rope fibers) and tar to seal gaps between the planks of wooden ships. If they didn't seal every crack properly, seawater would pour in and the ship might sink. Today's caulk works on the same principle but is much easier to use.

When you caulk something, you're applying this sealant. A plumber might caulk around a new sink installation. A homeowner might caulk drafty windows before winter to keep cold air from sneaking inside.

Good caulking is invisible work that prevents big problems. A tiny unsealed crack might not seem important, but over months and years, water seeping through can cause mold, rot, and serious damage. That's why careful caulking matters more than many people realize.