Cells are individual units that make up the tissue (“stuff”) of each organ, like bricks form a wall. However, each cell is alive: it reproduces to make new cells as it eventually dies. So the skin, blood, muscle, etc. that we have today is not the same actual substance we were born with, but our genes make sure that each new cell of a body is just like the ones before & after it.
Each type of cell looks different: skin cells are rectangular, blood cells round, muscle cells oblong, nerve cells weird. Cells have their own little organs inside, called “organelles,” which perform functions like generate energy, make proteins, recycle waste, repair damage, etc.