In order to demonstrate aliasing, make a plot of the signal
x(t) = 3 cos 2π10t - cos 2π30t
which approximates a square wave with W = 30 Hz. If the sample points are taken at
t = 0, 1/60, 2/60, ......... 6/60
corresponding to Ts = 1/(2W), you can see that x(t) could be recovered from those samples. However, if the sample points are taken at
t = 0, 1/40, 2/40, ......... 6/60
corresponding to Ts = 1/(2W) and fs < 2W, a smooth curve drawn through these points will show the effect of aliasing.