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First, you can use the special keyboard shortcut that’s available in OS X Mavericks and later, which is Command-Control-Spacebar.
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I think there are a couple of easier ways to add those characters in, however, at least in places where there isn’t an obvious option of how to do so (like Mail). That’ll put a little square icon at the top-right of your screen, and when you click it, one of the options will let you show and pick from the available Emoji and symbols: You could also turn on the “Show Keyboard, Emoji, & Symbol Viewers in menu bar” option within System Preferences > Keyboard (under the “Keyboard” tab). You can, of course, click on the Emoji button that’s provided in Messages: Under OS X, there are several different ways to insert them.
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Those cutesy little characters, though sometimes annoying, are at least a good way to impart sarcasm or emotion, especially because it’s difficult sometimes to tell the tone of text conversations. I use Emoji all of the time (and no, I’m not actually 12 years old, despite what my friends may think).
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