A shopkeeper sold 3 blue pens today and 0 red ones

Why

The shopkeeper has real data — today, blue sold and red did not — but three sales is a tiny sample, and a tiny sample can point almost anywhere by chance. Maybe one customer needed three blue pens for a group and that was the whole day's pen business. Maybe the red pens were behind the blue ones and nobody saw them. Maybe tomorrow four people ask for red. With numbers this small, a run of one colour tells you very little about what people generally want. Test it: think about flipping a coin just 3 times — getting heads all 3 times is not rare at all, yet nobody would conclude the coin cannot land tails. His son is right: 3 sales is far too small to draw a conclusion from — you would need to track colour sales over many days before a real pattern showed up.