You will need a magnifying glass and a small piece of scrap paper for this demonstration.
a. Go outside on a sunny day and put the piece of paper on a patch of asphalt, concrete, or a large flat rock. Make sure there is no other combustible material or debris in the vicinity.
b. Hold the magnifying glass so it is close to the paper, with rays of sunlight shining through it. Move the magnifying glass side to side and close or away from the paper until the most concentrated ray of light is shining on the paper. This is the focal point for that magnifying glass.
c. Measure the distance between the focal point and magnifying glass. This is the focal length for that magnifying glass. (Be careful; if you hold the magnifying glass in that place for long enough, the paper will catch fire. Do not ever do this on your skin or clothing, as it can cause serious burns.)
d. Now take the magnifying glass inside and use it to look at the words in a book. What is the focal length now?