Data Validation

I need two drop down lists, the second being dependant on the chosen result of the first. For example, list one has three choices of materials, Sheets, Bars and Tubes. If "Tubes" is chosen, list two must only display the sizes of Tubes available, ditto for Sheets and Bars. Creating the lists is not a problem, but how can I make the second dependant on the first??
Your help greatly appreciated