The idea is that they turn from transparent to purple when your sun cream wears off, so you know it's time to reapply.
But while they may be an interesting mini experiment in checking how well you apply your sun cream or whether it's doing its job, the experts we talked to weren't convinced that UV stickers were necessary or worth it for good sun safety practice.
Here's what you need to know.
Eat well, live better and stay healthy:How UV stickers work
UV stickers contain a skin-mimicking layer with photosensitive dye that reacts to UVA and UVB light, and a layer of adhesive to keep the sticker in place.
The UV layer is purple in the presence of UV light. When you stick it on to clean skin and apply sunscreen over it, you should see it change from purple to clear.
When the sunscreen starts to wear off and is therefore no longer blocking UV, the sticker should turn purple again, indicating that it's time to reapply more sun protection.
This is supposed to help you to keep track and not get caught out by forgetting to reapply your sunscreen.
– our independent lab tests reveal the bottles to buy and the ones to avoidShould you use UV stickers?
Cancer Research UK's Dr Julie Sharp told us: 'There isn't reliable evidence to show that UV stickers help us stay safe in the sun. Cancer Research UK doesn't recommend using them as we don't know whether they reduce the chance of being sunburnt and these products aren't tailored to different skin tones.'
It can be hard to keep track of when it's time to reapply sunscreen, especially with kids. However, generally speaking, none of us apply it often enough – and don't use enough – so always err on the side of caution. It can be helpful to set an alert on your phone to remind you.
Advice is generally to reapply generously and often, at least every two hours, and after activities such as swimming or where you get sweaty, paying special attention to frequently forgotten or more exposed areas.
– if you're finding it tricky, watch our video to find out the best way to apply sun protection to keep your little one safeProven ways to stay sun-safe - and common mistakes to avoid
Dr Sharp says: 'Too much exposure to UV radiation from the sun is the main cause of skin cancer, but we can take steps to reduce the risk.' These include:
Choose a good sunscreen. Spend time in the shade. Cover up. Don't be fooled by cloud. Don't forget to stay hydrated. undefinedsource https://www.which.co.uk/news/article/uv-detection-stickers-are-they-worth-buying-a8oFm4d5RqU3