Final answer:
Caffeine is thought to relieve headaches because it is a vasoconstrictor. It constricts the blood vessels in the brain, reducing blood flow and relieving headache pain.
Step-by-step explanation:
Caffeine is thought to relieve headaches because it is a vasoconstrictor. When caffeine is consumed, it constricts the blood vessels in the brain, leading to a decrease in blood flow and reducing headache pain. This is especially effective for migraines, which are often associated with dilated blood vessels in the brain.