The nature of miracles performed by an Avatar is sometimes not understood even by learned pandits and expounders of Upanishads and Gita. But, let us see if the Bhagavad Gita itself gives us any direction as to the nature of an Avatar.
The Gita tells us that God takes over a human form for establishing Dharma on a sound footing. The form God takes over is called an Avatar and thus any miracles the Avatar performs are acts of God. Is not the taking over of the human form a miracle of the highest order? God as we know is the Creator, the Preserver and the Destroyer. Thus, whatever the Avatar creates or does is but the natural act of God, though we call it a miracle.
God as we know is Omnipotent, Omnipresent, Omniscient; so is an Avatar. Sri Krishna tells Arjuna, I know all beings, past as well as present, nay, even those who are yet to come, but none (devoid of faith and reverence) knows Me." If therefore, Bhagawan Sri Sathya Sai Baba tells you your past, present or future, you consider it a miracle though He considers it His natural behaviour.
Then again Sri Krishna says, Who-so-ever offers Me with love a leaf, a flower, a fruit or even water, I appear in person before that disinterested devotee of purified intellect and delightfully partake of that article offered by him with love". This shows the Avatar's omnipresence. Bhagawan Sri Sathya Sai Baba too appears at a devotee's house hundreds of miles away, though bodily present at Prasanthi Nilayam. We call this a miracle, though it is but His natural behaviour.
We have in the Gita an example of an Avatar's Omnipotence when He appears before Arjuna endowed with numerous arms, faces and eyes and having infinite forms extended on all sides; again when He is seen endowed with a diadem, mace and Chakra, in mass of splendour shining all round, having the brilliance of a blazing fire and sun, dazzling and immeasurable on all sides. If we believe in what is stated in the Gita we cannot, but come to the one and only conclusion that performance of miracles is as natural to the Avatars as eating and drinking is to us.
Sri Sathya Sai Baba tells us "Why did Krishna raise the Govardhana Hill and keep it aloft? It was to announce His Truth and His Nature, to install Faith and to implant Courage. It is just a sign as each one of My Acts is. There is no task which I cannot accomplish, remember no weight I cannot lift. You have faith in Rama and Krishna because of the books which describe a part of their achievements and the experience of the Sadhakas who attempted to delve into their mystery. You have not demanded direct proofs of Divinity from either Rama or Krishna. Have faith first and then you will get proof enough."
Let us remember that not even a millionth part of what Bhagawan Sri Sathya Sai Baba does to help His devotees or to alleviate their suffering is known to us or is on record. If you wish to experience His miracles or get His help, He needs only three things—Faith, Sincerity and Surrender. When these are forthcoming, His help is assured.