Whether you've been sight singing for years or are a novice in this field, there are a number of things that you probably didn't know about sight-singing. There are a lot of misconceptions out there about this skill, and we will discuss a number of them here. Chances are you'll come away knowing more about this field than you did before.
1) Sight singing takes much less musicianship than playing an instrument. This is an area that is commonly misunderstood. Most of the books and courses on sight singing assume that you have a thorough knowledge of music theory. Such knowledge is necessary for playing an instrument because you have to know exactly what note you are playing. But in singing, you only have to know the relation between the notes and the tonal center, a relative knowledge rather than an absolute knowledge.
2) Using solfege offers advantages Over using numbers. When you learn sight singing, you can either sing according to the numbers of the scale (1, 2, 3), or by using syllables of the solfege system (Do, Re, Mi). Although numbers are probably more intuitive, the syllables offer several decisive advantages. One number (sev-en) has two syllables, and thus doesn't fit the sing-by-number system. Numbers are used for many parts of music education, including rhythm, harmonic structure, chord positioning, and even dynamics. This makes use of numbers potentially ambiguous. Most importantly, numbers can't easily account for accidentals between the scale notes, while syllables can handle them easily.
3) Virtually all songs end on "Do". This is something that seems intuitive, but you may not have thought about it a lot. But this feature makes it easy to find "Do" in any tonality. Just imagine the end of the song, or even a phrase that could conceivably end the song. That final note is "Do". You can find everything else in relation to it.
4) Singing in a key with a lot of sharps or flats is just as easy as singing in C. This is something that is not generally realized. And it seems that most methods that teach sight-singing seem to start with music in C because it's easier for piano players. So what! When you're singing, all you have to know is which line or space represents "Do". Sure, you use the key signature to identify that note, but then, you don't worry about sharps or flats. Just sing the other notes in relation to "Do". It's so much simpler than playing an instrument.
5) Singing in strange clefs should not be an obstacle. Many people feel that they can only sight-read in, say, the treble clef. This may be true for instruments, but it's ridiculous for singers. You don't need to know the name of the note that you are singing, just how it relates to "Do". If you have a weird clef, you find "Do" by means of the key signature, the same as always. Then sing other notes in relation to that. The only time that might be a challenge is when there are no sharps or flats. In this case, "Do" is C. Since the weird clef is probably a C clef, the line in the middle of the clef symmetry is middle C. Problem solved.
Now that you know these five little-known facts about sight singing, you should find that you can do it (or teach it) with more confidence and accuracy. Or perhaps you now realize how little you really have to know about music to sight sing successfully. Best of luck as you continue to learn or teach this enjoyable skill.