The following experiment has lots of potential for further investigation and can be carried out at home or in the laboratory. Take two cups of water straight from the tap. In one cup drip a layer of light sewing machine oil onto the surface. Measure the temperatures in each. Shine a torch directly into each cup of water for five minutes. The surface layer of oil is noticeably warmer than the corresponding water in the other cup, and so the overall temperature of the oil-covered cup is slightly higher too. The hydrocarbons absorb infra- red radiation from the torch. This is analogous to the absorption of infra-red radiation by the atmosphere which is then reradiated and has a warming effect. Extensions could be to use different covers over the base of the cup such as silver foilblack paper or to alter the thickness of the oil.