 This video is about significant figures or why 2 cm and 2.0 cm are not the same. No matter what tool or method we use to measure something, there is always a limit on how precise we can measure. Significant figures are used to tell us what this limit is. The basic rule is only the last number that we write down is estimated. In this number here, 2 cm means the tool is actually estimated and we're not sure about it. If I write 2.0 cm, what I mean is I'm absolutely sure about the tool, but the zero, I'm not so sure. As another example, if I use a regular ruler with millimeter increments to measure the length of this block here, then I would get 8.99 cm. Why 8.99? Because I have millimeter increments, I can see very clearly, I'm absolutely sure about the 8 cm, I'm also sure about the millimeters, and then the last number is a guess. I was pretty close to 9 cm, so I guessed the 9 cm. So if I write it down in this form, I tell whoever is looking at my number that this one is a guess, those two numbers I'm sure about.