There are many amazing underground creatures. The stars of the underground show are earthworms,__16__ their role in building soil, their importance to gardeners, and the fascination they afford schoolchildren. Earthworms are giants compared with most other soil__17__ . The earthworm’s body is made of__18__ cylinders, which look like many little rings joined together. They are__19__ that chew their way through soil, feeding on plant and animal detritus and on microbes, digesting and thoroughly mixing organic and inorganic components before eliminating it as humus. This__20__breaks complex organic molecules down into simpler forms more suitable for uptake by plants.

        Mark is a young man with synesthesia. It is not a__36__ (although it sounds like one), and it does not really have any serious effects on his day-to-day life. However, it is a strange condition. Synesthesia happens when two or more of a person’s senses get __37__ . For example, Mark “tastes” words. His sense of taste__ 38__  when he hears or reads certain words, even if he is not eating anything. For him, the word “box” tastes of eggs.

       His sister is synesthetic, too, and she sees words__39__ color. Hence, when she sees the word “Tuesday” or just thinks of the word “Tuesday,” she gets the feeling of “brown.” Actually, the kind of synesthesia, where the days of the week are colored, is the most common type. Some scholars suggest that synesthesia is__40__  to the way our brains develop language and that there is a link between sounds and shapes.