[header] How to demonstrate the greenhouse effect [title] Place the thermometers in direct sunlight. [step] Put them on a concrete or wood surface that is in direct sunlight outside. Allow them to sit in the sun for 3 minutes. 
[substeps] Set the timer or clock to 3 minutes so you know when the time is up. You can also place the thermometers under a sun lamp for 3 minutes if you are creating a model indoors.