Secret of Miracles
The whole universe itself is a miracle, which is the modification of His mind called Maya. The mind is His energy or power. This power is the substance that pervades all over the world. This Maya (mind) is the water and all the objects of the world are only waves in this Maya. Since the Maya is inseparable from the Lord, you can say that the Lord (Brahman) is modified into the world. When you are day-dreaming and you imagine a dream-city, you are not modified into the city.
Only your mind is modified. As your mind changes, so does the dream-city. Therefore according to Maya, anything can take place in this world. The Maya is under the full control of the Lord. The wish of the Lord itself is Maya. All the miracles are only the wishes of the Lord. The Lord may wish to follow certain rules, in such case, there is no miracle. But when the Lord wishes to break some rules, a miracle happens.
Therefore, miracles are not the real characteristic signs of the Lord and the power to do them is transferable from the Lord to anybody. The miracles are only associated characteristics of the Lord like ornaments or jewels. The Lord may exhibit His jewels or may remove and hide them. Lord Krishna exhibited miracles and Lord Rama did not exhibit.
A cow should be recognized only by the characteristic loose skin hanging below its neck, which is not present in the case of any other animal. Similarly the Lord must be recognized only by His characteristic signs like Jnana, Prema and Ananda (knowledge, love and bliss).