Step 1: You have an allergy. Your body releases 'histamine' molecules that act like keys. Step 2: These histamines find 'H1 receptors' on your cells and unlock them, causing a runny nose. Step 3: You take an 'Antihistamine' tablet. Step 4: The antihistamine drug molecules enter your bloodstream. Step 5: They reach the H1 receptors and plug them up. Step 6: Now, the body's natural histamine molecules cannot find a spot to sit because the drug is already blocking the 'keyhole'. Step 7: Since the receptor isn't triggered, you stop sneezing.