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What is this component?

I found an electronic component and didn't know what's it for (no serial or product code on it). But one thing I know: When you have no education in electronic engineering, the best way to learn is to try out and see what happens.

I assembled the ??? on the arduino breakoutboard and looked at the analog readings in the serial monitor. So it is a sensor. And values change when I cover it with my hand.
Guess it's a light sensor. yeaj.

[Code in progress...]