Grab a ruler. Draw a 3 cm line. There you go!
Scroll down to related links and look at "Inches and centimeters in comparison".
On a typical metric ruler, the major divisions are centimeters, with smaller marks at intervals of 0.1 cm (1 mm), and a medium-sized mark at the halfway point between centimeters. So 0.2 cm or 2 mm is two of the smallest intervals on the ruler.
4 inches are 10.16 centimeters.
Just like snow - with a ruler, in inches or centimeters
A ruler
Here you find inch and centimeter rulers. Scroll down to related links and look at "Inches and centimeters in comparison".
You should get a ruler and start measuring things in your home that look like they'd be two centimeters.
The customary ruler uses both inches and centimeters.
a 30cm ruler
It look like 1.93
You use a ruler or similar object that is marked with centimeters. If your ruler only has inches, measure the inches, then multiply with 2.54 to get centimeters.