sentences: a disease causes bones become weak and brittle & to become something is a kind of change ->  a disease causes bones to change  a disease causes bones to change & if something causes something else to change, then that something affects that something else ->  a disease affects bones  a disease affects bones & skeletal system is made of bones ->  the disease affects parts of the skeletal system  the disease affects parts of the skeletal system -> the disease affects the skeletal system