 Often, it is necessary to count the number of significant figures, there are some rules with this. First of all, any number that is not a zero will always count. So here in my 123, I would have one, two, three significant figures. Then zeros are a bit more complicated. Zeros do count if they are sandwiched in between other numbers. For example, if I have one, zero, zero, one, thousand and one, the two, zero, zero sandwich, so they count, so here I have four significant figures. The zeros also count if they're to the right of a number and the decimal point. So one point zero, zero, these zeros count three significant figures. They would not count if there is not a point on the right here. So if there is not a point on the right, they would not count. In this case, I have hundred with one significant figure. If I add a point here, then my zeros actually count and I have a number with three significant figures. So there's a difference in writing hundred and writing hundred point. The zeros do not count if they're on the left of any number. Like here, this one doesn't count because there's no number here. So in this case, I have only the three, one significant figure plus the numbers on the right of it and on the right of the decimal point, so two significant figures. As a comparison, if I would have had one point zero, three, zero, then that zero suddenly counts because it is sandwiched between numbers, there is a number on the left, so I would have one, two, three, four significant figures. And as I wrote before, hundred without a point would only be one significant figure. Now why do we need to count significant figures or be able to count significant figure? First if we convert from one set of units into the other or if we convert into scientific notation, just by converting it, we are not increasing or decreasing the precision of the measurement. So we should keep the same amount of significant figures. For example, if I convert one hundred to scientific notation, I would write one times ten to the two. Why? One hundred. There's no point here, so these series don't count, so I only have one significant figure. So in my number here, only write one significant figure. If I had one hundred point, then that would convert to one point zero, zero times ten to the two. One, two, three significant figures, giving me three significant figures here.