my Select Case function is not working correctly

I have a simple list of items in one column and quantity per item in the other column. I needed to create a custom function that would pick up the quantity per item and automatically assign a percentage discount per item based on the quantity of that item.

Function Guide(Quantity)
    Select Case Guide
        Case Is <= 5
            Guide = 0
        Case 6 To 10
            Guide = 0.1
        Case 11 To 15
            Guide = 0.15
        Case 16 To 20
            Guide = 0.25
        Case 21 To 30
            Guide = 0.3
    End Select
End Function

The function is supposed to pick up by looking at the quantity in the original column and automatically assign a discount based on the quantity for that item. But when I run the function...it's giving me zero for all answers! What am I doing wrong?