As you keep your mind focused on the rims of your nostrils, you will be able to notice the sign of the development of meditation. You will feel the pleasant sensation of a sign. Different meditators experience this differently. It will be like a star, or a round gem, or a round pearl, or a cotton seed, or a peg made of heartwood, or a long string, or a wreath of flowers, or a puff of smoke, or a cobweb, or a film of cloud, or a lotus flower, or the disc of the moon, or the disc of the sun.
The sign!
