The adder is a snake found in Britain and some other countries.
One day a poor adder found himself in the company of a well-to-do grass snake. "How long have you been living in these parts?" asked the adder.
"Well," said the other snake. "I lived two years under the apple tree, two years under the elm tree and a year under the oak tree."
"That means you've lived here seven years," said the adder.
Why did he say that?