Here’s what Professor Hallux has to say:
The easy answer is that it’s because of the huge amount of bacteria you find in poo. There are trillions of them in your gut – some are harmful germs, but most are very helpful in breaking out the goodness from the food we eat. Many of these bacteria release chemicals called sulphurous compounds – and they give poo its pooey smell.
It might not be very nice but it’s actually useful for us that poo smells – it serves as a biological warning to keep away from it, or anything contaminated with poo. If you’ve trodden in dog poo, you’ll soon know it without having to see it! And scientists think this helps to keep us safe.
You see, in nature most animals will also stay away from their own poo – which suggests that they think it smells too!
And that’s why you should always wash your hands after going to the toilet!