In this day and age, protecting a child's skin from sun exposure is vitally important. I had orginally received a sample of these little stick on sensors with a pack of diapers. They worked so wonderfully that I went out and purchased more (marketing at work).
These little fish shaped stickers, fade with UVb exposure. When first applied, the fish have a sun pattern on them. As the level of exposure increases, the suns blend into the background. When the suns are no longer visible, you know it is time to reapply sunscreen. I for one, am a horrible judge of how long it has been since I last did something, so having a visual clue is very helpful for me.
These stickers are water resistant and can be applied to skin or clothing. I have used them on hats, sleeves, or skin. I usually put them on my daughter's back where she can't see or mess with them.
I found that when i have my sunglasses on, it can be hard to tell when the pattern has faded, but a quick peek over the top of the glasses clues me in.
These are a great way to help reduce the risk of sunburn and UVb Exposure.