In phonetics, aspiration is the strong burst of breath that accompanies either the release or, in the case of preaspiration, the closure of some obstruents. In English, aspirated consonants are allophones in complementary distribution with their unaspirated counterparts, but in some other languages, notably most Indian and East Asian languages, the difference is contrastive.
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In dialects with aspiration, to feel or see the difference between aspirated and unaspirated sounds, one can put a hand or a lit candle in front of one's mouth, and say spin [spɪn] and then pin [pʰɪn]. One should either feel a puff of air or see a flicker of the candle flame with pin that one does not get with spin.