Today’s Wonder of the Day was inspired by Hayleigh. Hayleigh Wonders, “How do things glow in the dark? ” Thanks for WONDERing with us, Hayleigh!

Are you afraid of the dark? It's OK to be a bit scared of things that go bump in the night. But what about things that glow at night?

You've probably seen or played with glow-in-the-dark toys before. From yo-yos and balls to stickers and stars, glow-in-the-dark toys can turn any dark room into a magical world of adventure!

Have you ever WONDERed how these things glow in the dark? Guess what? It's not magic! It's just plain and simple science.

Glowing in the dark — also known as luminescence — simply requires chemicals that store energy when exposed to light. These special substances are called phosphors. This type of glowing is sometimes called phosphorescence.

Phosphors radiate visible light after being energized. This means you have to expose the items to light for a while before they will glow in the dark.

Phosphors then slowly release their stored energy over time. As they release the energy, they emit small amounts of light, which we see as an object glowing.

Sometimes glow-in-the-dark objects will only glow very weakly for a short time. Often, you have to place them in a very dark place to see their faint green glow. Newer glow-in-the-dark items may glow more brightly for several hours.

Over the years, chemists have created thousands of chemical compounds that act as phosphors. For glow-in-the-dark toys, manufacturers look for phosphors that can be energized by normal light and that glow as long as possible.

To make glow-in-the-dark toys, manufacturers mix their chosen phosphor into plastic and then mold it to the desired shape. Two of the most common phosphors found in glow-in-the-dark toys are zinc sulfide and strontium aluminate.

There are a couple of other types of luminescence. Chemiluminescence, for example, makes object glow in the dark because of a chemical reaction. When two particular chemicals react, they produce energy that is subsequently released, creating a glow. This is what happens in glow sticks.

Radioluminescence uses phosphors that are constantly charged by adding a radioactive element, such as radium, to them. You may have seen this type of luminescence on the hands of a watch, for example.

One final example from nature is bioluminescence. Some creatures, such as fireflies and jellyfish, contain chemicals within them that cause them to glow. Some of these creatures glow for protection, camouflage or to attract mates.

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Be sure to get some good shut-eye before you read tomorrow's Wonder of the Day!