Explain why the invention of the microscope led to the discovery of cells.

Cells are very small and cannot be seen with the mere naked eye so the discovery of the microscope, which magnifies small objects, allows us the see those cells. 

Here is a not very in depth answer...
before the microscope we couldn't see into something as small as a cell. the invention of the microscope let people see objects in a much smaller break down. If you were to look at a plant leaf with a naked eye, you would just see the characteristics of a plant leaf. The microscope made it possible to see what made up that leaf, cells.

(hopefully this helps!)

The density of water is 1 g/cm cubed and in order for the pebble to sink its density has to be greater than the density of water and in order for the twig to float its density has to be less than the density of water

Sorry i dont understand the question can you rephase it?