Everybody likes pleasant odors—the perfume of flowers, or the aroma of a cake baking. But a disagreeable odor can be quite unpleasant. The strange thing is that the amount of odor in both pleasant and unpleasant cases can be very, very tiny! Did you know that we can detect the odor of certain substances if only one part of 30,000,000,000 parts by weight is present in a given weight of air? And this is true of man—whose sense of smell is a fraction as good as that of a dog!