By Jacquie Buttriss, retired primary teacher

I was teaching five and six-year-olds and wanted to do a sequence of lessons about the properties of water - evaporation, condensation and dissolving. One of these key scientific concepts was displacement and I wanted to focus on Archimedes’ principle.

The first group gathered around the table and the children took turns to pour water into the bath, about two-thirds of the way up the side. One child marked the level of the water. We then went through the story - I decided to use Mr Archimedes’ Bath, by Pamela Allen. In the book, Mr Archimedes has a bath with three of his friends - Kangaroo, Wombat and Goat. The problem is that, whenever they have a bath together, the water always overflows. Mr Archimedes decides it is time to find out who the culprit is, so they all take turns getting in and out of the bath and Mr Archimedes measures the water.

We used stones for my lessons, which we painted as the book’s characters.

First, the Mr Archimedes stone was put into the bath. Then each of the other characters - Kangaroo, Goat and Wombat - was carefully placed in the tub. Finally, the level of the water was measured.

This couldn’t be right. There was no discernible difference and I had to admit that the water was still at the original level marker. My assistant and I exchanged bemused glances and the children asked, predictably, “Can we do it again?”

“Right everybody, stop what you’re doing, find your pebbles and come over here to join in.” I thought that if it didn’t work with one set of stones, it was sure to work with two, or maybe three. So, we tried again, placing the two biggest Mr Archimedes stones in the bath, followed by the two biggest Kangaroos, Goats and Wombats. “Right, stand back everybody and we’ll see how much the water has gone...” I looked at the line. I looked more closely at the line. The children crowded in to see for themselves.

“The water’s gone down,” said one, knowingly. “I knew it would,” said another.

My assistant’s smile broadened as she stood up and pointed to the side of the bowl. There was a tiny, hairline crack in the side. The children’s observations skills were spot on. The more stones we put in the bath, the more the weight of the stones made the water leak, imperceptibly, out of the tiny split.

* Chalkface Lovesongs is broadcast on Radio 4 on Sundays at 5.40pm, starting on February 9, and repeated the following Saturdays at 7.45pm.Jacquie Buttriss reads her story on February 9 (repeated February 14). Catherine Lloyd reads ‘A Timeline for Billy’ on February 16 (repeated February 22)