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Unit 4F is about the difference between insulators and conductors. You can simply place an item to be tested across the contacts and see whether the LED lights up.
Metal things such as coins, keys, and paperclips conduct.
Rubber is a classic insulator.
The wooden part of a pencil is an insulator.
The lead is both a conductor (the wire down the middle) and an insulator (the plastic covering).
You can use the lead to connect various things across the contacts.
Note that people are not insulators. If you have your fingers across the contacts you will probably light the LED anyway.