I burned the back of my knees once. Just the backs of my knees. I had applied sunscreen everywhere else — arms, face, shoulders — but missed a two-inch strip behind each knee. I could not walk comfortably for three days. That is when I realized I had been applying sunscreen wrong my entire life.

Most People Use Half the Amount They Need

The SPF rating on the bottle is tested at 2 milligrams of sunscreen per square centimeter of skin. In real human terms, that is about a shot glass worth (1 ounce) for your whole body. Most people use a quarter of that. A nickel-sized dollop for your face alone, not a pea-sized dot. If a bottle of sunscreen lasts you all summer, you are not using enough.

The Forgotten Spots

• Ears — tops and backs
• Back of the neck
• Tops of feet
• Backs of knees
• Scalp part line (spray sunscreen or wear a hat)
• Eyelids (use a mineral stick — it will not sting)

Reapply or It Does Not Count

Sunscreen breaks down after about two hours. Sweat, water, and just existing in the sun degrade it. If you put it on at 10 AM and are still outside at 2 PM with no reapplication, you have essentially no protection. I set a timer on my phone now. It is annoying but so is skin cancer.