Fun fact: sun protection factor (commonly known as SPF) isn't really about the potency of your sunscreen, it actually refers to how long your sunscreen will last and protect you from UVB rays. Other factors that affect this include the amount applied (half the sunscreen = half the protection) and your environment, (water and sand being your strongest opponents). The general rule is one teaspoon per body part and reapplying every 2 hours or if you go in water when in the sun. But skin experts advise that you invest in a daily sunscreen if you want to keep your skin looking youthful.