This octave/MATLAB script shows the effect of not sampling fast enough.

Two sinewaves fit the same sample points. (There are actually an infinite number that fit. Can you figure out the next higher frequency one?)

% This program demonstrates how a couple of sine waves can be
% sampled and have the same sample values.
clf;
f=1;
tmax =5;
T = .8;
fs = 1/T; %1.25 Hz sample rate
t=0:.01:tmax;
x = cos(2*pi*f*t);
hold on;
tk=0:T:tmax;
xk = cos(2*pi*f*tk);
y = cos(2*pi*(f-fs)*t);
plot(t,x,'b',tk,xk,'ro',t,y,'g');
legend('First Signal','Data Samples','Second Signal');
title('Aliasing in the Time Domain');
xlabel('Time (seconds)');
ylabel('Signals');
figure(1);
hold off;