What are insulators?

1 What are conductors of electricity? 2 Name some materials that are good conductors of electricity. 4 Give some examples of insulators. 5 In previous class you have used a tester as shown is figure given below to test whether electric current passed through a particular material or not? When this tester is used for liquids sometimes even though the liquid allows the electric current to pass and the circuit is complete, bulb does not glow. What can be the reason?