paragraph: Both conduction and convection transfer energy through matter. Radiation is the only way of transferring energy that doesnt require matter. Radiation is the transfer of energy by waves. These waves can travel through empty space. When these waves reach objects, they transfer energy to the objects. This causes them to warm up. This is how the Suns energy reaches Earth and heats its surface. Radiation is also how thermal energy from a campfire warms people nearby. You might be surprised to learn that all objects radiate thermal energy. This even includes you. In fact, when a room is full of people, it may feel noticeably warmer. This is because of all the thermal energy the people radiate! 
question: What form of energy do a lot of people in a room radiate?
answer: Thermal energy