Caulk developed into its modern form during the 1940s and '50s, when it started selling in disposable tubes like the ones used today. Although many types of caulk are now available -- from painter's caulk, to silicone types, window and door caulks, and many others -- they all serve the same purpose of sealing cracks and stopping leaks. This useful product does have its drawbacks, however, and that is its ability to show dirt and suffer mold and mildew growth.
Put on protective gloves and then soak some toilet paper or paper towels in liquid chlorine bleach.
Wad the paper into narrow pads a bit wider than the caulk itself.
Place the pads over any mildew spots, and leave them there overnight. If the caulk is a total mess -- around a bathtub rim, for instance -- then run the pads along the entire perimeter of the tub.
Remove the pads the following morning and rinse the areas clean.