PUZZLEBOXX

You could use a special measuring tool, called a MICROMETER, which is designed for measuring thicknesses extremely accurately.

OR

Using an ordinary centimetre rule, measure a wad of paper 1 cm thick. 

Count how many sheets make up 1 cm of paper.

EXAMPLE

125 sheets = 1 cm

therefore

1 sheet = 1/125 cm

which is the same as

1 sheet = 0.008 cm

which is the same as

1 sheet = 0.08 mm