Why do Olympic-level swimmers wear two caps?

En route to her gold-medal performance in the women's 100-meter butterfly race, U.S. swimmer Dana Vollmer's outer cap fell off, showing another swimming cap underneath it.

Many Olympic-level and other competitive swimmers, including Michael Phelps, wear two caps during races. There are a couple of reasons for this, according to Dave Salo, assistant coach of the U.S. Olympic women's team, head swim coach at the University of Southern California, and personal coach to several top Olympians.

First of all, the double cap ensures that goggles stay on. Swimmers put on the initial cap, then their goggles, and then place an additional cap on to secure the goggles strap.

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A second reason for the double cap is that the pairing of specific materials helps reduce drag, the resistance the body encounters as it moves through the water. The first cap is generally a latex cap, which stays on the head better but also wrinkles, while the second cap is usually made of silicone.

"The outer silicone cap better maintains the shape and does not wrinkle as much, thereby causing less drag," Salo told Yahoo! Sports.

Still, the double cap isn't for everyone. Salo noted that a few of his swimmers, including USC star and Team USA member Haley Anderson, prefer to race with a single latex cap.

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