Final answer:
Leyina's observation is explained by the metal spoon being a good conductor of heat, which allows it to warm up quickly when stirring hot chocolate.
Step-by-step explanation:
Leyina's observation that the metal spoon heated up when stirring hot chocolate with marshmallows can be explained by the fact that the spoon is made of a good conductor of heat. Metals are generally good heat conductors because they contain many free electrons that can transport thermal energy. Contrastingly, materials like plastic and wood are poor conductors of heat and are often used as insulators. Since the metal spoon is a good conductor, heat spontaneously flows from the hotter hot chocolate to the cooler metal spoon, resulting in a more uniform distribution of thermal energy as the spoon warms up.