It is harder than Euclid if you don't know beforehand.

But you might get a grown-up person to explain it to you with books or wooden bricks.
I will tell you what a pen is because that is easy.

It is the bit of river between one lock and the next.

In some rivers 'pens' are called 'reaches', but pen is the proper word.
We went along the towing-path; it is shady with willows, aspens, alders, elders, oaks and other trees.

On the banks are flowers--yarrow, meadow-sweet, willow herb, loosestrife, and lady's bed-straw.