As a new Mathematica user I am puzzled by the following, where the Part[list, n] function - or more accurately list[] - doesn't seem to have consequent results. For example, in MM 9 on Windows 7 if I say:

f[x_, y_] := (x + y)^2

curves = Expand[Thread[f[{1, 2, 3}, {x, y, z}]]]

bounds = {{x, -10, 10}, {y, -10, 10}, {z, -10, 10}}

Plot[curves[[1]], {x, -10, 10}]

this works, but Plot[curves[[1]], bounds[[1]]] returns an error even though bounds[[1]] returns {x,-10,10}. Anbody know why this is?