Story Time 5.6C

I was out walking with a friend when we came to a stream which was pretty wide (about 10 metre). It had been raining quite a bit in the previous few days and the stream looked full. I wasn't too sure about whether we could cross here. Perhaps we should look for an easier place farther down-stream. My friend would have nothing of it. It doesn't look too deep to me. I'm going to take my boots off, roll my trousers up and wade across. And so he did. In the very first step he lowered himself into the water from the bank, slipped and found himself almost waist deep in the stream. What happened??