Each piece has a slot (the groove) cut all along one edge, and small outcrop (the tongue) on the opposite edge. Two or more pieces thus fit together.
... out with 22.3% under the age of 18, 6.3% from 18 to 24, 31.0% from 25 to 44, 26.9% from 45 to 64, and 13.5% who are 65 years of age or older. The median age is 40 years. ...