What must happen to the temperature of a material for thermal expansion to occur.

What is the scientific definition of energy that relates to work

This is for my 8th grade science class, i really don’t understand. can someone me? , lol.

Which of the following correctly describes what happens when an atomic bomb explodes? small pieces of fissionable material are joined and form a body with a mass greater than the critical mass, the relative number of neutrons escaping decreases, and a chain reaction and explosion result. large pieces of fissionable matter are brought together quickly and form a body with a mass smaller than the critical mass, the relative number of escaping neutrons increases, and a chain reaction and explosion result.