A simple, and popular, demonstration of the greenhouse effect involves a higher temperature being observed in a container with an elevated concentration of CO[subscript 2] inside than in a container with just air enclosed, when subject to direct light. The CO[subscript 2] absorbs outgoing thermal radiation and causes the air inside the container to be warmer. However, in some variations of this experiment an additional positive effect can arise from artefacts in the experiment, such as the slightly heavier CO[subscript 2] forming a layer at the bottom of the container and suppressing convection. Therefore, the physics of this demonstration is elucidated in a system that does not suffer from such artefacts. In particular, the absorption of infrared radiation due to the enclosed CO[subscript 2] is measured, and a one-dimensional model of heat transfer is solved. It is found that the temperature of the enclosed air is significantly higher inside the container with an elevated concentration of CO[subscript 2] inside, but that the temperature of the container itself is not appreciably higher.