You type Alt+= or click the Insert ribbon Equation button, and presto! You’ve inserted an empty math zone place holder that states “Type equation here.” in the language you’re using. Then you type a^2+b^2<space> and you see a2+b2, except in better typography. The empty math zone place holder seems simple and intuitive, but behind the scenes a fair amount of software is handling a variety of situations that probably don’t come to mind when you do the things above.
For example, if you cursor away from the math zone, either by using the arrow keys or the mouse, the empty math zone place holder remains; it’s just not selected anymore. If you save the file and reopen it, you’ll see the place holder just where you inserted it. If you select the place holder, possibly with text surrounding it, and copy paste it somewhere else in the document, you’ll see it pasted at the target location.
Furthermore if you return to the place holder, either using the arrow keys or the mouse, the place holder will automatically be reselected. So if you then type something, what you type replaces the place holder. Yet another characteristic: if you type Backspace in a math zone until the last character is deleted, the empty math zone place holder reappears and is selected. Naturally if you type Backspace yet one more time, the place holder is deleted, just as it is if you type the Delete key.
Now what if you type Alt+= immediately before or after an unselected empty math zone place holder? You guessed it! Instead of inserting a new place holder, the current one is reselected. There are other variations that Word 2007 doesn’t get quite right: if you select a bunch of text including one or more empty math zone place holders and then type Alt+=, you’d probably expect the included empty math zone place holders to be deleted, since they’re inside a math zone and nested math zones aren’t permitted. But Word 2007 doesn’t catch this relatively unlikely scenario (it doesn’t crash either, thank goodness!)
The bottom line is that the seemingly simple idea of having a place holder for inserting some math isn’t as straightforward as you might expect.
What do to in the future? The empty math zone place holder is clearly a useful math user interface feature, even if it is tricky to implement correctly. In addition, particularly for educational purposes, one might like to have other place holders within nonempty math zones to ask the user to type, for example, the numerator of a fraction or the integrand of an integral. Such place holders also could be copied and saved to file and reselected by moving the insertion point into them.