These students are eating the radish seedlings that they grew in a previous experiment.  They are tasting the three parts of the seedlings separately: first the cotyledons, then the hypocotyl, and finally the tap root
     This activity is called Sun Salad because it is a lesson on photosynthesis, and it's a lead-in to upcoming lessons about food chains and webs. The students will be asked to explain how the radish seedlings enabled them to "eat" the sun's energy, and what would happen to animals if plants disappeared from the earth.
     The hypocotyl was a hot like a radish--A little too hot for some!--because that's the part the becomes the radish.  The students had to infer this from their observations.