The maths in rung 2 is easy. What costs marks is forgetting the three rules that keep it honest.
Trap One: It Lives Between 0 and 1
A probability can never be negative and never go above 1. 0 means impossible, 1 means dead certain, and everything real sits on the line between. If you ever calculate a chance of 1.5 or −0.2, you haven't found a weird probability — you've made a slip. The "drop a chance on the line" toy makes this concrete: every value lands somewhere on that 0-to-1 track, never off the ends.
Trap Two: You Can Only Count If It's Fair
The "favourable ÷ total" shortcut only works when every outcome is equally likely. A fair die is fine — each face is just as likely. But you can't say "rain or no rain, so it's 1/2," because those two aren't equally likely. When outcomes aren't even, counting them lies; you need data instead. Always ask: are these slices the same size?
Trap Three: Small Samples Fool You
Switch to the "small vs large samples" tab. A fair coin is theoretically 0.50 heads, but flip it just 10 times and you might get 7 heads — 0.70. That's not a dodgy coin; small experiments are just noisy. Hit "flip 1000" and the experimental line crawls right back to 0.50. Experimental ≠ theoretical for small samples, but the bigger the sample, the closer they get.