Something is added

Erklärus had brought a little tin full of glow-stones – round, warm stones that glowed faintly in the dark like tiny stars.

He put three of them on the table.

Erklärus: “Look. Three glow-stones.” Then he put two more with them. “And now two more have been added. How many are on the table now?”
Liri: “I’ll count: one, two, three... four, five. Five!”
Erklärus: “Exactly. That was an addition. Something is added to something else – and you count how much it is altogether.”
Liri: “I can do that with stars too!”

🌌 Adding stars

In the belt of the constellation Orion, three bright stars shine close together. Next to them shine two more bright stars.

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3 + 2 =

💛 Sweetheart numbers – pairs that make 10

Some pairs of numbers like each other especially: together they always make exactly 10. Mathematicians call these number bonds; Erklärus calls them the “sweetheart numbers”.

1 + 9= 10
2 + 8= 10
3 + 7= 10
4 + 6= 10
5 + 5= 10

Now you: which number is missing to make the pair 10?

3 + = 10
6 + = 10

🚀 Bridging through ten – on to 20

Sometimes 10 isn’t enough – then you carry on to 20. The trick: first top up to exactly 10, then add the rest.

Erklärus: “7 rockets are already on the launch pad. 5 more are added. How many is that? First I work out 7 + 3 = 10 – then 2 of the 5 are left over. 10 + 2 = 12.”
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🟡 7 rockets were already there  ·  🔵 3 more top up to 10  ·  🟢 2 last ones are added after that – 12 altogether!

7 + 5 =
🌌 Addition in the universe

In Orion’s belt there are 3 bright stars; in the constellation Cassiopeia there are 5.
4 planets in our solar system have rings (Jupiter, Saturn, Uranus, Neptune).
So far 6 crewed Moon landings have succeeded – 12 people in total have walked on the Moon.

➕ Exercise 1
Cassiopeia has 5 bright stars. In a neighbouring constellation, 4 more shine. How many altogether?
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➕ Exercise 2 – Bridging through ten
8 satellites are already orbiting a planet. 6 more are launched. How many are there then? (Hint: top up to 10 first!)
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🌌 Exercise 3 – On the Moon
So far 12 people have walked on the Moon. If a new rocket takes 5 more astronauts there – how many would that be altogether?
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What remains

Liri had lined up all the glow-stones in a row. Five, twelve, seventeen – numbers that had only been words before suddenly made sense.

Liri: “Addition is like meeting friends. There simply get to be more of you.”
Erklärus: “That is the loveliest definition I have ever heard.”

Liri wrote the new calculating trick onto her star map – one more memory on the way home.

“Addition: addend + addend = sum. Sweetheart numbers make 10 together. When bridging through ten, first top up to 10, then add the rest.”
What you learned today
Addend + addend = sum
Sweetheart numbers: always 10 together  ·  Bridging through ten: first up to 10, then the rest
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Chapter 3 mastered!

From hands-on counting to mental arithmetic – Erklärus and Liri can now add up to 20.

Erklärus’s notebook – page 3:
“Addition: addend + addend = sum. Sweetheart numbers: 1+9, 2+8, 3+7, 4+6, 5+5. Bridging through ten: first top up to 10, then the rest. Liri learned it with glow-stones – with her hands, before she could do it in her head.”
← Back to overview 3 – Addition, Part 1