It might be picked up from a trip to the beach

SAND

Think of what often gets stuck between your toes or clings to a wet towel after a day by the shore.

This clue plays on the double meaning of "picked up." Literally, you might collect sand while visiting the beach, but "pick up" can also imply catching something like a cold or a souvenir. Here, it’s simply referring to the grains of sand you’d naturally gather on a beach trip.