It may very well be that you do not. But If you use Microsoft Word for Windows you have this function. In most cases this function is not installed then installing Word. But with the install CD you can install it later.
The OP asked for open source, so that rules out using Word.
TeX and LaTeX are perhaps most well-known for this task; go here to learn more. The tools are powerful, but they can be a bit intimidating to install and use on your own without spending some learning time.
A number of years ago I found Open Office and liked it so much that I dumped Microsoft Office -- and that was a bit of an effort, as I had used Word for more than 20 years and had lots of macros and tools. But OO's math capabilities, in my opinion, are superior to Word's and much easier to use (if you know TeX, it will look somewhat familiar). Here's a brief introduction to some of the capabilities.