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Chapter 2 · Ages 9–11

Momo and
the Big Number

In the last chapter, you practiced with a coin and a die. Now let's look at why it matters whether you flip 10 times or 1,000 times – and what weather forecasts actually mean.

Big Numbers, Part 1

A few flips can lie to you

Flip a coin just twice, and you could easily get heads both times – that would look like 100% heads! Only when you flip a lot does the truth show up.

Click the buttons and watch how the share of "heads" changes as more flips add up.

–
No flips yet

The more flips, the closer the bar gets to the middle (50%). This is called the law of large numbers: with few tries, chance stands out strongly; with many tries, it evens out.

Two Dice

Some numbers are more likely

Roll two dice at once and add up the total. Sounds just like one die – but it isn't!
Total: –

Roll often and watch which bars get tallest. A 7 can happen six different ways (1+6, 2+5, 3+4, 4+3, 5+2, 6+1) – a 2 only one way (1+1). That's why 7 is much more likely than 2 or 12, even though each single die number is equally likely on its own.

In Real Life

What does "30% rain" mean?

On the weather forecast you often hear things like "30% chance of rain tomorrow." That doesn't mean it rains 30% of the whole day – it means something else.

Imagine there were 10 days that all started out exactly like today. With "30%," it actually rains on about 3 out of those 10 days – and not on the other 7.

30%

Move the slider and see how many of the 10 raindrops turn "on." The higher the percentage in the forecast, the more of ten similar days actually bring rain.

You did it!

What you know now

  • ✅ With few tries, chance can be very misleading.
  • ✅ With many tries, the true pattern shows up (law of large numbers).
  • ✅ Some totals with two dice are more likely than others.
  • ✅ A percentage in a weather forecast describes many similar days, not a single one.

In the next chapter, it's all about charts that look honest at first glance – but aren't always.