They made their way through thick acacia bush, following a rough twin-tracked road. The earth was red here, and there was little vegetation to hide it, with only the acacia trees providing a note of green. The track curved and then dipped down towards the lake that, though diminished by the dry season, was still home to a small family of hippos and flocks of water birds.

Neither spoke much on this walk, although Gwithie stopped at one point and drew Mma Ramotswe’s attention to a plant by the side of the track.

‘Look at this,’ she said. ‘I have a soft spot for this plant. It has quite a few Setswana names but the one I use is kgaba. Do you know it?’

Mma Ramotswe tried to remember. Her father had shown her plants when he had walked through the bush with her, and he had taught her the Setswana names, but she had had difficulty in remembering them. The old words, people said, were slipping away, remembered only by a handful of elderly people. The world as described in Setswana was becoming smaller and smaller with each year that passed. Gwithie, she knew, was working on a book of wild flowers and had gone out of her way to learn the old names before they were lost.

She peered at the plant. Like everything else it was struggling, and a thin layer of red dust had coated its leaves. But she thought she knew its blade-shaped leaves, and she nodded.

Gwithie reached down to touch the plant gently, as a doctor might touch a patient. ‘People use this for a variety of complaints,’ she said. ‘Like almost everything in the bush, it has its uses. This is good for arthritis and rheumatism, apparently. And you can also eat it as a sort of spinach.’ She straightened up, smiling as she spoke. ‘It can also be used to treat children who fail to look after their parents,’ she continued. ‘And to bind together the members of football teams.’

Mma Ramotswe laughed. ‘It must be a very busy plant,’ she said.

They continued on their walk. Now the track drew near the edge of the lake and she knew that this was the spot. Mma Ramotswe had been there on that sad day, and she remembered.

They stood before the rock, a natural boulder that had been used by rhinos as a rubbing stone. She saw the smooth parts of the stone where, for generations and for centuries, the animals had rubbed their hide. These were their landmarks, the monuments of animals that had once been plentiful; now, in many places, only the stones remained. As she stood there, she recalled a fragment of a story that her father had told her a long time ago, just a line of it: Our brothers, the rhinos, who are gone now.