Eleven Through Nineteen Understood as Ten and Some Extra Ones

You will meet this idea in ordinary days. A dozen eggs plus a few extra in a bowl is a teen number sitting on a real kitchen counter. This is worth learning because seeing the ten inside teens makes every later place value idea easier.

Watch for it today, and when you spot it, tell someone what you noticed and how you knew. Ask yourself where else the same idea might be hiding, then go and look.

Watch this carefully, friend. Every teen number is one ten and some ones. Fourteen is one ten and four ones. Seventeen is one ten and seven ones. Build them and see: one rod plus four cubes is fourteen.

This is why teen numbers all begin with a 1 when you write them. That 1 is not one. It is one ten. Once you see the hidden ten, teens stop being strange and start being predictable.

Watch out for eleven and twelve. Their names give no hint at all that a ten is hiding inside them, unlike sixteen or seventeen. Build them with rods and cubes so you can see the ten that the names refuse to tell you about. If you get stuck, use your fingers or counters. Good mathematicians use tools.