The Twin Miracle
The Twin Miracle(Yamaka Pātihāriya) is a miracle(supernormal activity or Ṛddhi) which can only be performed by a Buddha. Any other beings including ascetics, gods or Brahmas can't do the Twin Miracle. Lord Buddha did this miracle to show a proof of the Buddha-hood. Also he did this miracle to take attention of many people to stop a war.
According to the 'Anagata Vamsa', when the Buddha doing Yamaka Pātihāriya with his psychic powers, he produce flames of fire and streams of water from all pores of the body simultaneously. When one position produce water, next position produce fire and positions change alternately. This happen in all pores. Here are some examples as in the 'Anagata Vamsa'. When a stream of water produce from left nostril then a flame of fire issue from right nostril then in the next movement a flame of fire issue from left nostril and a stream of water produce from right nostril. When finger tips produce streams of water then finger toe produce flames of fire and change similarly. When pores of the skin produce streams of water then hairs of the skin produce flames of fire and change similarly. But water and fire will not mix.
A higher meditator can produce different kind of things by attaining 4th Jhana with the relevant Kasina meditation subject(Kammaṭṭhāna). But only one type at a time. Only a Lord Buddha can produce many types at the same time by switching the meditation subject instantly.
1. "Anagata Vamsa," edited by J. Minayeff in Journal of the Pali Text Society (1886)