Two of you stand up. Count every finger you can see together. How many fingers altogether?
Could we count them a quicker way than one by one?
Look at two ten-frames on the board. Here is 19: a full ten and nine more, with one empty space left.
Now watch what happens when that last space fills and both frames are full.
Both frames are full. That is two full tens.
The 2 at the start of 20 means those two tens. The 0 means no loose ones left.
Look at the number 20. Why do you think it starts with a 2?
We will build a few numbers together on the board, one at a time. We will do 15, then 17, then 19, then 20. Each time, say how many spaces are still empty.
Build 15 to 20
In your maths copy, write 19 and 20. Then draw two quick tens for 20: two simple bundles or two boxes of ten.
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