If you bite into a lemon - it gets sour. The body doesn't care how acidic it is inside. The so-called pH value, a measure of acidity, is adjusted very precisely in the blood, otherwise we end up in a life-threatening situation. The reason for this is that if the pH value is incorrect, the building blocks in the blood and the body's cells function too quickly or too slowly or are destroyed. Breathing and the kidneys ensure that the pH value is set exactly right. The correct pH value is also often of decisive importance in many laboratory experiments. See what simple and uncomplicated means can be used to determine the pH value. You and your children can try it out for yourselves.