Sunscreen is a lotion, cream, or spray that protects your skin from the sun's harmful rays. When you spread sunscreen on your arms, face, and legs before going to the beach or playing outside, you're creating an invisible shield that blocks or absorbs ultraviolet light (called UV rays for short). These rays can burn your skin, cause painful sunburns, and damage skin cells over time.

The sun emits different types of UV rays, and good sunscreens protect against both UVA and UVB rays. You'll often see sunscreen labeled with an SPF number: SPF 30, SPF 50, and so on. SPF stands for “sun protection factor,” and higher numbers mean stronger protection. SPF 30 blocks about 97% of UVB rays, while SPF 50 blocks about 98%.

Scientists developed the first sunscreens in the 1930s and 1940s, and they've improved dramatically since then. Modern sunscreens work in two ways: some contain minerals like zinc oxide that sit on top of your skin and reflect UV rays away (you might have seen lifeguards with white cream on their noses), while others contain chemicals that absorb UV rays before they can damage your skin.

Sunscreen needs to be reapplied every couple of hours, especially after swimming or sweating, because it gradually wears off or washes away.