Alex tickles his brother by stroking adjacent ________ spots on his skin.

A) pressure
B) warmth
C) cold
D) pain

Tickling is often caused by a light touch or stroke on sensitive areas of the skin, such as the underarms, sides of the torso, or soles of the feet. These areas are particularly sensitive to pressure, making option A the best choice. The other options, warmth, cold, and pain, are not typically associated with the sensation of tickling.