My mother used to correct me all the time when I was younger about "bringing" a toy to a friend's house versus "taking" it. Which is correct, and why?
Your mother is right.
It boils down to knowing the definitions of the words. The definition of bring from dictionary.com is:
Bringing with involves coming to.
Taking with involves going away from.
When you were talking to your mother, your were going away from her. That is why it was correct to say that you were taking something with you.
If your were talking with your friend over the phone, it would be correct to use bringing, because you were going toward him.
Bringing or taking is relative to the (future, present, or past) presence or absence of a person at the destination to which something shall be, is, or has been brought or taken. This person is not the bringer/taker.
It could be a person to whom the bringer/taker tells what bringing or taking he/she shall do, is doing, or did.
It could also be a person who tells to a third person what bringing or taking the bringer/taker shall do, is doing, or has done.
If the person shall be, is, or was present at the destination while the bringer/taker shall be, is, or was on his/her way, bring is the appropriate word.
If the person shall be, is, or was absent from the destination while the bringer/taker shall be, is, or was on his/her way, take is the appropriate word
One possible answer is that you "bring" things to the place you are now, and "take" things to the place to which you're going.
So you "take" the toy to your friend's house, but once you're there you can be said to have "brought" the toy along with you. It's pretty subtle though, I don't think it's an especially crucial difference!
ETA: I've just thought of a good example. Suppose you are in a room with your toy. Your mother could tell you "take it over there!" and then "bring it back over here!"... but the other way round would sound pretty odd.
It's all a matter of perspective. It boils down to what's in the speaker's head (i.e., his perspective). For example, if my focus is on myself, I might say "I'll bring the beer with me." However, if our focus is on the party and its location, I might say, "Are you going to take beer?" I bring something along with me, but I take something to the destination.
In english the original meaning has been lost due to users bad use, so know it doesnt "sound" incorrect to say so, eventhough it is.
But if you take spanish as a example you will see that the difference between taking and bringing is crutial and cant be exchanged.