A glass tube is best, but a straw will do very well.

Put a little pure water into one glass and the same quantity of lime-water into the other glass.

Now put one end of the tube into the mouth and place the other end in the pure water.
Breathe through the tube a few times.

Look at the water in the glass and see that no change has taken place.

Now breathe through the lime-water in the same way.