Look it up in the dictionary

Here is one thing that I don't get about some teachers.  If you ask them about how to spell something they say "Look it up in the dictionary" .  The dictionary was meant for looking up definitions, not for spelling.  What if I have no idea how to spell a word like xylophone.  Sounds like it has a z in it.  When you ask that teacher what a word means they will probably explain it to you instead of saying "Look it up in the Dictionary".  No idea why.