Build a circuit using a photoresistor

What can we use instead of hard-to-find parts like the photoresistor, 100 ohm resistor, or alligator clips?

If you can't find a photoresistor (LDR) use a small phototransistor or a light sensor module, if you don't have a 100 Ω resistor use a higher value such as 220–330 Ω to still protect the LED, and if you lack alligator clips follow the instructions to twist wires around the LED legs and secure with electrical tape or use a coin cell instead of a battery holder.

My LED doesn't light—what should I check first while building the circuit?

Make sure the longer LED leg is attached as the positive anode per the instructions, that the wire from the anode goes through the resistor to the battery positive and the cathode goes through the photoresistor to the battery negative, that all clips/twists are tight and taped, and that the battery has charge and the photoresistor is being illuminated when you expect the LED to brighten.

How can we adapt this activity for different ages?

For younger children have an adult pre-strip wires, handle battery connections, and demonstrate covering the photoresistor to show the LED dimming, for grade-schoolers let them twist and tape the wires with supervision, and for older kids move the same connections onto a breadboard and use a multimeter to measure how the photoresistor changes with light.

What are simple ways to extend or personalize the light-sensitive circuit?

Add a small toggle switch between the resistor and battery so you can turn the circuit on/off, wire two or three LEDs in parallel to increase brightness, or mount and decorate the components inside a shoebox to make a personalized nightlight to photograph for DIY.org.