Why do we sweat? Why do our fingers wrinkle in water? Why do we get sunburns? Joe Hanson explains it all in this summer science episode of It’s Okay to Be Smart. Bonus: find out what those SPF numbers mean on your sunscreen.Related videos about how our bodies work: You Are Your Microbes, The Beautiful Physics and Math of Sneezes, The Mystery of Motion Sickness, the classic Schoolhouse Rock Telegraph Line (Nervous System), and How Much Stuff Do Our Bodies Make In A Year?