Where do new cells come from?

New cells come from old cells.  When the time is right, an animal cell or a plant cell divides into two, forming two new cells.  These are called daughter cells.  The two new cells are exactly the same as the original cell.  This process is called cell division.

You may be thinking, "But that's not right - they should have half of what was in the original cell!"  Although a cell may be to small to see, its not stupid - before it divides it makes an extra copy of everything in its nucleus.  This means that the two daughter cells get a complete nucleus.  This is important because the nucleus contains the 'recipe' which is used to tell the cell what to do, including telling it how to divide to make new cells.  They do share the cytoplasm but they can make more and end up the same size as their parent cell.