How do I type greek letters in Yahoo! Answers like "theta", "sigma" etc. ?

There are a couple of ways you can do it.

My preferred way is to use the predefined HTML entities for them. This is pretty easy: just put & in front and ; at the end, like this:

θ becomes θ

For capitals, just capitalise the first letter:

Σ becomes Σ

There are plenty of others you can use, not just Greek letters, for instance ± for ±, √ for √ and so on. Wikipedia has a good list, see the link below.

You can also use the Character Map utility (assuming you're on Windows; other systems have similar utilities) to find it and copy and paste it. I find this to be slower, though.

Use a Windows-native program called Character Map. Do a Search for it on your computer.

Alternately, use a table of unicode characters like the one below.

I keep Microsoft Word open. I use Insert / Symbol to get the symbol I want. Then I copy and paste into Yahoo! Answers.

I saw an article the other day about this, let me check my history real fast and I will get back with the link

If you have a Mac, download a program called ukelele. It lets you define characters that can be typed with two keystrokes.