A kalpa is a time period which can be defined most clearly through metaphor. Suppose there is a huge rock extending 100 miles on every side. Every 100 years a heavenly maiden comes down to the earth, rubs the rock once with the soft sleeve of her dress, then flies back to heaven. Another hundred years later, she does the same and repeats this journey every one hundred years until the rock disappears. The length of time necessary to cause the rock to disappear is a kalpa. It is extremely long period of time. In pre-Lotus Sutra teachings, one could attain Buddhahood by means of “Ryakkō-shugyō,” practice for many kalpas.