* You can demonstrate the effect of light exclusion simply by laying an old door on the grass, but if you have the imagination, you can create something intrinsically interesting. Work with the children to make a design or pattern that consists of simple outlines with white space in between. This could be linked to other work the children are doing, such as the sunflower shapes created at the Coombes. Alternatives are enlarged leaf shapes - Sue Humphries recommends oak leaves arranged in a spray, one for each class - or mazes, which then open up a whole new play opportunity. Large, repeating shapes are most effective - try 1m oak leaves, or a 10m sunflower.
* Transfer the design on to opaque material such as black polythene, old carpet or anything that will block the light. Parents can help transfer the children's designs and cut out the shapes - use a craft knife for carpet, cutting from the back.
* Choose a grassed area that receives full and equal sunlight right across the space. The grass needs to have been mown regularly to give a clear design, although it may grow quite a bit while the effect is developing. Spread out the design and anchor the pieces with tent pegs, skewers, stones or bricks.
lCheck for colour changes every week and decide when would be the best time to remove the cover. Three weeks is a good deadline to aim for. The colour sequence starts with green and moves to yellowy-green, to yellow, to white, to bare earth. Even if the grass is completely killed, it will quickly regrow. This brings another opportunity for learning. As Sue Humphries says: "The joy comes after the event - watching how quickly nature is able to recover."
Rosemary Ward is a writer for 'Gardening Which?'