These purple stickers should be applied directly to the skin and under the sunscreen of your choice with the help of hypoallergenic adhesive. After about 15 minutes in the sun, the stickers will turn clear, an indication that your sunscreen is working. After about two hours, the sticker will gradually start to turn purple again, a sign that your sunscreen is losing its protective abilities and that it’s time to reapply.

According to the company, the stickers work by using a patented technology to mimic the surface of the skin and the ways that it interacts with the sun’s rays. Each sticker lasts a total of 12 hours, and is resistant to water and sweat and won’t leave a tan line. We’ve previously covered these clever stickers before and received some guidance from dermatologists if you’re interested in finding out more.

Get them from Amazon for $16.49+.