Automatic light

 In the circuit above, the battery voltage is shared between the resistance box on the left, which was set to 1000 Ohms, and a light dependent resistor on the right, clipped to the blue board. With light shining on the LDR, its resistance is much lower than 1000 Ohms so it gets less than half of the battery voltage. That's not enough energy to light the LED which is clipped across the LDR.

I put my hand over the LDR to make it dark. The resistance shot up and it got more than half of the battery voltage, enough to light the LED.