My guacamole turns brown in the refrigerator. How can I prevent this?

Fruits and vegetables such as avocados, bananas, apples, peaches, pears and potatoes contain enzymes that cause their flesh to turn brown when exposed to air. Although the discolored flesh is not harmful or even perceptible by taste, it doesn’t look very appetizing.

The easiest way to keep guacamole from browning is to prepare it just before serving. Also, add a little lemon or lime juice to the mixture if the recipe doesn’t already call for it: Acids slow the browning process. If you want to make guacamole ahead of time, squeeze extra citrus juice over the surface. This creates a barrier, reducing the amount of oxygen to which the dip is exposed. To store, press plastic wrap onto the dip’s surface and refrigerate for up to two days.

Despite these precautions, some browning will still occur if guacamole is left for several hours or more. Scrape off the top layer before serving to reveal the vibrant green dip below.

There is moss on our roof. How can we get rid of it?

Make sure the growth is moss, not algae. Look down from an upstairs window, inspect it from a ladder, or ask a roofing company to examine your roof for you: If the green material looks like a thin film of paint, it’s probably algae, says James R. Kirby, senior technical director for the National Roofing Contractors’ Association.

Moss is thicker, with stem and leaflike protrusions, and tends to build up in the crevices between shingles. While algae is harmless, many experts believe that moss damages a roof by trapping moisture, which deteriorates wooden and asphalt shingles. The plants’ rootlike structures may also penetrate roofing materials, breaking them apart over time.

A roof-cleaning company can remove moss (or algae) with a power washer. But this process may dislodge surface granules on asphalt, making the roof susceptible to damage from ultraviolet light. It also may wear down wooden shingles. Applying a moss-eradication product is another option. Choose one that contains soap salts, as these break down rapidly in soil. Administer the products with a garden sprayer, or hire a roof-cleaning company to do it.

Since moss thrives in moist, shady conditions, you can take steps to prevent future growth by clearing your roof of leaves and trimming back any overhanging tree branches. If the problem returns, a roofer can install zinc or copper strips near the top of the roof. Rainwater floods over them, covering the roof with small amounts of the metals, which prevent new moss from growing. Because zinc and copper often find their way into rivers and streams, where they can be toxic to fish, avoid this treatment if you live near a body of water.

Finally, when it’s time to reroof, consider switching to metal. This surface, which is coated to prevent harmful runoff, is too slick for moss to grow on.

How do you remove a nail polish spill from a carpet?

As with all spills, it’s important to act fast, before the stain dries. Gently blot – don’t rub – the spot with a clean, dry, white cloth or a paper towel, working quickly to prevent the stain from spreading. Continue blotting, shifting the cloth so that you’re using a clean section each time, until the cloth is no longer absorbing liquid.

Use a plastic spoon to scrape off any polish that has partially dried, then dab the edges of the spill with a cotton ball dampened with a non-oily acetone nail polish remover that doesn’t contain any conditioners; blot with the cloth. Working from the outside in, continue dabbing with remover and blotting until no more polish comes off. (Test a discreet area first; if the remover damages the carpet, skip this step).

Next, mix a quarter teaspoon of dishwashing liquid with a cup of lukewarm water. Apply it to the spot with a clean, dry, white cloth; blot and repeat, as above. When no more color comes out, thoroughly sponge the area clean with warm water, and blot to dry. For stubborn stains, or if you have delicate rugs, contact a professional carpet cleaner.

Questions should be addressed to Ask Martha, care of Letters Department, Martha Stewart Living, 11 W. 42nd St., New York, NY 10036. E-mail to mslletters@marthastewart.com.

2007 Martha Stewart Living Omnimedia Inc.