Miracle fruit is a small red berry that contains a protein called miraculin, which can temporarily alter the perception of taste. When the fruit is consumed, miraculin binds to taste receptors on the tongue and makes sour or acidic foods taste sweet. This effect can last for up to an hour, and can make foods like lemons, vinegar, and grapefruit taste like candy. Miracle fruit is often used as a natural sweetener and flavor enhancer, and is also being studied for its potential as a treatment for diabetes and obesity.