The statement which explains why muscle cells and skin cells do not look and act the same is this: THE GENES IN EACH TYPE OF CELL ARE BEEN EXPRESSED DIFFERENTLY.
All the cells in human's body has the same set of genes, this is because, all the cells in the body originated from a single fertilized egg. Even though all the cells in the body has the same origin, they are different from one another because, the cells use the same set of genes differently.
Earth is always rotating around the sun that means if it were winter on one side of the world, (that is not on the side of the sun) the other side of the world would be summer, ( the side that the sun is shining on)
Carbon shares 4 electrons to complete its valence shell