Pronouncing it with two syllables is idiomatic, or has to do with the dialect of English you speak. Some people actually pronounce it SMI – yuhl. But it actually only has one syllable.

The fact that some say “one” and some say “two” MAY be based on how they pronounce it in their own dialects, but very likely that is NOT the case here.

Rather, both groups are trying to describe something that does not quite fit our usual categories.

What no one has noted is that there are certain sounds that are DIFFICULT to make in succession, which can lead speakers to add a very slight unaccented vowel sound (called “schwa”). This sound is so slight, and usually passed over so quickly. Sometimes it becomes so distinct that most or all would agree that it creates an additional syllable. But when that does not happen, it is hard to know how best to describe/count the number of syllables.

Frankly, I think this demonstrates that the system of breaking words into discrete “syllables”, though generally very useful, does not always function perfectly in describing how we actually speak. Note the “syllabification” (breaking words into syllables) is a description or analysis of what we actually do — a TOOL we use, not a RULE that precedes the creation of words and to which our speech must therefore submit! It can become artificial, forced to do what it cannot. The case you bring up, and others like it, are good examples of this.

As for the sound combinations where this difficulty arises —

In English, the problem is typically found when a two-vowel-sound combination (or “diphthong”) is followed by a ‘lateral’ sound –/l/ or /r/

Just try a bunch of words that have the sounds /ou/ /oi/ and /ay/ (=sound of the “long i”) followed by /l/ or /r/ vs. the same combinations with OTHER consonant sounds.

“mine” or “mite” are easily pronounced as one syllable; “mire” and “mile” are not so easy, seeming to force a subtle half-e sound before the final sound

Compare “out” vs. “our” and “owl”, and “coin” vs. “coil” (there are no English words ending with -oir in which the oi is pronounced as the oi/oy diphthong).

Look up some of these words in the dictionary and you’ll see what I mean. Words that all standard dialects pronounce as homophones or perfect rhymes may, in the dictionary, be listed as having different numbers of syllables (e.g., “roil” is written as having one syllable, “royal” as having two, cf. “our” vs. “shower”). OR a dictionary entry may list BOTH a one-syllable and two-syllable pronunciation for the same word

check these out for a start:

http://dictionary.reference.com/browse/owl

http://dictionary.reference.com/browse/towel

http://dictionary.reference.com/browse/roil

http://dictionary.reference.com/browse/royal

Another illustration of all this is in songs — a word like “power” will, in some songs take two distinct notes and be treated as having two syllables; in others it takes one note, and is pronounced as having one syllable.

One final note — syllabification of a word may change when you add sounds/endings. For instance, if you add -ing to make your word “smiling”, the /l/ combines with the ending to form the second syllable, and matters are not so difficult any more.

Smile has one syllable. I teach my young students to check the number of syllables by clapping their hands as they say the word. How many times can you clap your hands when you say smile? One time.

How many times can you clap your hands when you say elephant? Three times.

Americans tend to pronounce it /smarl/

while in the UK it’s /smy uhl/

You pays your money and takes your choice.

you don’t count the silent ‘e’ at the end as a syllable.