Starch and cellulose are polysaccharides but you can eat or digest starch and not the cellulose why?

Both starch and cellulose are polymers of glucose. However, the individual glucose units are linked differently in the two. Humans have an enzyme which is capable of breaking the linkages used to form starch, but do not have one that can break the linkage used to form cellulose. (If you want the technical terms, cellulose uses a beta(1-4) link and starch uses both alpha(1-4) and alpha(1-6) links.)