Seedling
First collect some empty plastic cups as seen in the picture. Get some cotton and a piece of goes. Fill half of the cup with cotton and lay some Bengal grams or peas. Just put one layer of goes peice on top of the seeds. Now keep watering regularly to keep the cotton wet and help the grams / peas to grow. Children can easily understand the process by daily observation of the seedling progress.
Try this and explain them importance of keeping our earth green.
Hope it is useful for you.
