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. 

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 pin [pʰɪn] and then spin [spɪn]. 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. In most dialects of English, the initial consonant is aspirated in pin and unaspirated in spin. 

In the International Phonetic Alphabet (IPA), aspirated consonants are written using the symbols for voiceless consonants followed by the aspiration modifier letter ⟨◌ʰ⟩, a superscript form of the symbol for the voiceless glottal fricative ⟨h⟩. For instance, ⟨p⟩ represents the voiceless bilabial stop, and ⟨pʰ⟩ represents the aspirated bilabial stop.
How can you tell an aspirated sound from an unaspirated one?
what is one way to do that?
Is there another way?
what are two words you can speak to notice the difference?
which is aspirated?
what is the definition of aspiration?
what happens to a candle when one speaks an aspirated sound near it?
What does IPA stand for?
How are aspirated sounds noted?
Where is the modifier placed?
test for a puff of air
one can put a hand or a lit candle in front of one's mouth, and say pin [pʰɪn] and then spin [spɪn]. On
put a candle in front of your mouth
one can put a hand or a lit candle in front of one's mouth, a
put your hand in front of your mouth
one can put a hand or a lit candle in front of one's mouth, and say pin [pʰɪn] and then spin [spɪ
pin and spin
say pin [pʰɪn] and then spin [spɪn]. 
pin
he initial consonant is aspirated in pin
the strong burst of breath
 strong burst of breath that accompanies either the release or, in the case of preaspiration, the closure of some obstruents
there will be a flicker in the candle flame.
er feel a puff of air or see a flicker of the candle flame 
International Phonetic Alphabet
n the International Phonetic Alphabet (IPA), a
by voiceless consonants followed by an aspiration modifier
sing the symbols for voiceless consonants followed by the aspiration modifier letter ⟨◌ʰ⟩, a superscript form
as a superscript
modifier letter ⟨◌ʰ⟩, a superscript form o