In this case – Thermodynamics
Hot – cause material to expand. Use a hair dryer!
Cold – cause material to contract. Use air duster – can of compressed air or actual air freeze!
This method will not work with all kinds of dents and materials. But in many cases you can experience the ‘material’ wanting to go back to its original shape.
If the paint doesn’t look damaged, it could still be prone to chip or ‘fault’ over time. But that happened with the denting in the first place…….