The speaker, who worked for a printing company, wanted to demonstrate why humidity is so much worse in hotter weather than in colder weather.
He placed two glasses on a table, one a tall drinking glass, the other a shorter juice glass. He put a sign in front of each indicating the temperature they represented: the tall glass represented 90 degrees, the shorter one was 65 degrees. He poured water into the shorter glass, filling it to the top.
“This is 100% humidity in 65 degree weather,” he explained. Then he poured the contents of that glass into the taller one, bringing it to about two thirds full. “But notice that the same amount of moisture is not 100% humidity at 90 degrees. Hotter temperatures hold more moisture, therefore making the relative humidity much greater.”