Counting, Comparing, and Combining: Building Number Sense to 20
Grab 12 crayons and 9 crayons. Which pile has more? How many would you have if you put them all together? You don't need to be a grown-up to answer these — you just need number sense, and that's exactly what we're building today!
What You'll Learn
- How to count objects one at a time all the way to 20 without skipping - How to tell which of two numbers is bigger or smaller - How to add two small groups together to find the total - How to take away (subtract) from a group and count what's left
Counting Carefully to 20
When we count a pile of objects, the most important rule is: touch one object for every number you say, and never touch the same object twice. Try it with 15 buttons or blocks. Line them up in a row so it's easy to keep track. The last number you say when you touch the last object is how many there are in total — that's called the total count.
More, Less, or Equal?
If you have a pile of 8 apples and your friend has a pile of 5 apples, you have more. We can check this two ways: by counting both piles and comparing the numbers (8 is bigger than 5), or by lining the apples up in two rows, side by side, and seeing whose row runs out first. The row that runs out first has less. If both rows end at exactly the same spot, the amounts are equal.
Combining Groups: Addition
If you have 6 stickers and your friend gives you 4 more, how many do you have now? You could count all 10 from scratch, but there's a faster way: start at 6 and count up 4 more — 7, 8, 9, 10. This is called counting on, and it's how addition works. The answer, 10, is called the sum.
Taking Away: Subtraction
If you start with 9 grapes and eat 3 of them, how many are left? Start with your total, 9, and count backward 3 times: 8, 7, 6. There are 6 grapes left. Subtraction is just the opposite of addition — instead of joining groups together, you're taking a smaller group away from a bigger one.
It's okay to use your fingers, blocks, or small toys to count! Every mathematician started by counting real objects before doing it in their head. The objects help your brain build a strong picture of what numbers mean.
If a bowl has no apples in it, we say there are zero apples. Zero is a real number — it just means none at all.
You have a row of 6 red blocks and a row of 9 blue blocks lined up side by side. What can you say about the two rows?
You have 8 crayons. You give 3 of them to a friend. Which method correctly finds how many crayons you have left?
The Snack Count Challenge
Gather two small snacks you have at home (like crackers and grapes), with no more than 20 of each. First, count each pile out loud, touching one at a time. Second, decide which pile has more and which has less. Third, combine both piles together and count the total. Fourth, eat 4 of the total and count how many are left! Deliverable: tell a family member your 4 numbers out loud — the count of each pile, the total, and what's left after eating 4.
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