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The Daily Cardinal Est. 1892
Friday, April 19, 2024

Ask Mr. Scientist


Dear Mr. Scientist,

How does sunscreen work, and what exactly does SPF mean?

—Sasha R.

Sunscreens work in one of two ways. The first is by filtering or even completely blocking the ultraviolet rays. Molecules that absorb UV light are able to release the captured energy as heat.  A second way is by reflecting the UV rays so they aren’t able to reach your skin, something achieved by using zinc oxide or titanium oxide. Regardless of how a product works, what’s important to pay attention to is something called the Sun Protection Factor or SPF. The SPF is a multiplication factor that tells you how much longer you can stay out in the sun before burning. If you normally get a sunburn without using sunscreen after 15 minutes in the sun, using a product with an SPF of 10 will allow you to stay in the sun 10 times longer—150 minutes.

Dear Mr. Scientist,

It gets pretty hot here in the summer and I’d like to go swimming, but how come the lakes here in Madison are always full of algae?

—Chris W.

As wonderful as Lake Mendota and Monona are, the massive algal blooms make it difficult to have any fun in the water. The biggest culprit behind these seas of green is element 15—phosphorus. The fertilizers used by farmers and homeowners contain large amounts of phosphorus. When it rains, water carries excess phosphorus farm fields and lawns into the waterways. The phosphorus, combined with warm summer days and sunlight, creates the perfect environment for algae to grow, oftentimes to uncontrollable proportions.

Dear Mr. Scientist,

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How do caterpillars turn into butterflies? What sort of magic happens inside its cacoon?

—Andy K.

The metamorphosis that occurs inside a caterpillar’s chrysalis is amazing. The caterpillar turns into a sort of cellular mush. The body breaks down into something called imaginal cells which, like stem cells, can become any type of cell. These imaginal cells reassemble themselves into wings, antennae and other body parts, but some parts, like the caterpillar’s legs, remain unchanged. All together the change from caterpillar to butterfly lasts about two weeks. Although the creature that emerges looks like a completely new insect, researchers have found that a butterfly retains memories of its previous life as a caterpillar.

Mr. Scientist is Michael Leitch. If you have a burning science question you want him to answer, e-mail it to

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