I was got one of those electronic project kits when I was 9 years old. One of the projects involved a light activated switch. Whenever light hit the sensor a circuit would turn on.
Another project involved making a circuit that emitted a horrible shrieking sound. My working theory is that one of the engineers involved liked created parental misery.
I figured out a way to combine the two. I taped a string to my door and on the other end I attached a piece of black cardboard paper to cover the light sensor.
When my door opened, the string would pull the paper off, exposing the light sensor and activating the shrieking sound. I had in fact invented an alarm system for my room.
I may have peaked at nine years old.