There are microscopic flying itch gnomes in the air. When a colony of them lands on you, you become itchy in that area. The gnomes (who actually glide, not fly) particularly like to land on people's scalps. By scratching you fling the gnomes in all directions and they have to regroup before they can attack you or another person again.
If a kangaroo is riding a blue bicycle down some railroad tracks on a Tuesday how many bottles of breast milk does it take to drown a jazz musician walrus?