An eggshell looks solid and is in fact pierced by thousands of pores t fine t see. Oxy moves inw through th and car dioxide mo out, s the emb breathes wit ever opening its mouth. Water vapour leaves by the same route, and the air space at the blunt end grows steadily as it does. A shell that lost water too quickly would dry the embryo out, and one that lost too little would leave no air to breathe at hatching.

Type only the missing letters. Each box is one letter, and the number of boxes matches the letters that are missing.