How do I call Server script from client script?

[MPardo]

If I have some server side script:
<SCRIPT Language="VBScript" RUNAT=Server>
Function DoSomething(sText)
....some code
DoSomething = sResult
End Function
</SCRIPT>

How can I call this code from something like a button click and pass data back from the server?

<SCRIPT ID=clientEventHandlersVBS LANGUAGE=vbscript>
<!--

Sub btnTest_onclick
...What is the code to run DoSomething and return a value?
End Sub

-->
</SCRIPT>