Sentence Completion

Complete the sentence with ONE word from the passage for each blank.

Caffeine works by tricking the brain. Normally a chemical called adenosine builds up during the day and makes us feel drowsy by binding to receptors. Caffeine has a similar shape, so it slots into those same receptors and blocks adenosine without activating them. The result is a temporary feeling of alertness. However, the body adapts over time, which is why regular drinkers often need larger amounts to feel the same effect.

Caffeine keeps people alert by blocking the chemical from reaching its receptors, and because the body adapts, regular drinkers may build up a —toleranceallergyshortage.