Everything in the universe is made up of atoms, those atoms are made of protons, neutrons, and electrons to make different kinds of atoms. The periodical table of elements is labeled with names of elements and the numbers of protons in the atoms that make up that element, the elements then fuse together to create things, for example two hydrogen atoms and an oxygen atom create water, a sodium atom and a chloride atom make salt and so on. In all of that they form cells, everything alive has cells, you, me, a blade of grass, tiny little bacteria, everything. A tiger is composed of cells in that same fashion, the atoms fuse together to create the carbon that life forms on earth are composed of, that carbon fuses with other elements and becomes a cell, no one really knows what causes life by we understand that cells are necessary, there are different cells that make different parts of that tiger, like the white and red blood cells. Those cells when grouped together make various organs, muscles, bones, etc. And those all come together to form the tiger, and that's how a tiger is made up of cells.