Help with Rich Text Boxes

[Coolness]

I have a really big Rich Text Box control on my form, named rtxMain. There are several procedures in my code that write text to this text box. Here is the simplest one:

Private Sub WriteStuff(ByVal stuffToWrite As String)
 
rtxMain.Text = stuffToWrite
 
End Sub

And later I would call that procedure:

WriteStuff("Hello, World!")

My question is, how do I write rich text (bold text, italic text, colorful text, etc.) to this text box, where I only want part of the text string to have special formatting? Like, if I ONLY wanted the "World" part to be bold, and the rest normal, how would I do this?

(of course that's not really what i'm writing, that's just an example)