Breaking up a string

Question

First, I would like to apologize if the answer to my question is common knoledge, but as I am new to C# it seems that I no longer have any common sence let allone common knoledge.

Ok, put really simply I need to know the quickest/best way to separate the first word in a sentence from the rest of the sentence, and have each stored in a different string.

For example,
Start with:

Sentence = "This is the sentence I want to split up."

and end up with

String1 = "This"
String2 = "is the sentence I want split up"


The reason I am looking for the quickest way to do this is that it is something the program I am working on will have to do constantly, so any little bit of time saved will add up quickly.