A physical property of any material is its density—its mass per volume. Pennies made after 1982 contain both copper and zinc. Pennies made before 1982 are pure copper. Zinc is less dense than copper, so post-1982 pennies are less dense and have less mass than pre-1982 pennies. Dig into your penny collection and find 20 pre-1982 and 20 post-1982 pennies. Measure their masses on a sensitive scale, such as a home postage scale. Alternatively, hold the pennies in opposite hands to see if you can feel the difference in their masses. How few pennies can you hold and still feel the difference? Try holding single pennies on your left and right index fingers. Can you tell the difference with your eyes closed? Try this with a friend.