Which process causes the handle of a spoon in a bowl of soup to become warm?

The handle of a spoon in a bowl of soup becomes warm primarily through the process of conduction. Heat from the hot soup transfers to the spoon's handle as the molecules in the soup collide with the molecules in the metal of the spoon, causing them to vibrate faster and increase in temperature. This transfer of thermal energy continues until the spoon reaches a temperature close to that of the soup.