How does the tornado form?

The tornado forms from the replacement of hot air by old air causing the air to spin and build speed till it has a funnel cloud made. It then slowly depending on the size of the tornado comes out of its wall cloud and either comes close or does touch the ground and causes much havoc on the area it has touched down upon. But it has been said that a tornado will touch down and then go straight back into its wall cloud to regain its strength so that it will be able to stay on the ground with enough force to cause destruction.