Glenn is heating water in a teakettle. The hotter water at the bottom of the kettle rises. The cooler water near the top skins and warms. Which kind of transfer of heat is this

Answer: Convection

Explanation: The heat transfer takes place by three modes:

1. Conduction: This type of heat transfer happens when there is direct contact between two objects.

2. Convection: This type of heat transfer happens when there is a movement of fluid (liquid or gas) in which convection currents are produced due to the movement of hot fluid upwards and cold fluid downwards. This happens as the density of hot fluid decreases and thus it moves upwards.

3. Radiation: This type of heat transfer happens when there is direct transfer of energy through space.

This is a pretty good detailed description of convection. Especially the parts about the rising and skinning.