Is there something I'm not getting here?

[Ruby The Roobster]

Following code doesn't work(it's not the actual code but it 
represents it). Is there some rule about function overrides that 
I don't know about?

class a {
public override string toString() {
//...
}
}

class b : a {
public override string toString() {
//...
}
}

The error I keep getting no matter what says: Error: Multiple 
Overrides of Same Function. Anybody know what I should do?