If the thermometers are too large to remain horizontal inside the jars, it is fine to stand them against an inner side. Every minute, for ten minutes, the students should record the readings of both thermometers. The air over the exposed thermometer is constantly changing, and as it gets warm it is replaced by cooler air. Because the air in the jar cannot circulate to the rest of the room, this air stays in the sunlight and gets warmer and warmer. A similar trapping of heat happens in the Earth's atmosphere.
Sunlight passes through the atmosphere and warms the Earth's surface.
The heat radiating from the surface is trapped by greenhouse gasses. Both the atmosphere and the jar allow light to enter, but then trap that energy when it is converted to heat. They work differently, however, because the jar keeps in the heated air, while the greenhouse gasses absorb radiative heat. To simulate global warming, the experiment can be done using two jars, one filled with air and the other with carbon dioxide. Every minute, record the readings of both thermometers without disturbing them.