The difference between 外 and 外側

I love learning languages, especially Japanese; I can't get enough of it. So I spend extra hours outside of class learning more words and reading beyond the reading assignments. When I was doing this week's worth of words, I came across 外(そと）and 外側(そとがわ). They're both 'outside'; what's the difference?

I talked to my teacher, and the main difference lies in their extent;

If you have a box, anything outside of that box is 外（そと）。The outside surface of the box, however, the top, bottom, and the sides, are 外側。

For example, if you say,

"I have gum stuck on the outside of my wallet,"

you would use 外側、and not 外：

財布(さいふ)の外側にガムが張(は)り付(つ)いた。

If you want to say that the dog is outside (not in the house), then you would use "外”　and not "外側。”