You lose your left sandal during a hot afternoon at the beach. Thus, you must hike back to your vehicle with one bare foot across the burning sand. Why does the bare foot get so much hotter?

The barefoot is so much hotter because the sun's rays are heating up the sand and the sand is becoming very hot and if you walk on the hot sand with no protection on your tender feet, it will get burnt. The other foot has protection from ythe very hot sand.