I was reading in my chemistry book about significant numbers. Also about measured and exact numbers. From what I understand, a measured quantity can be different than an exact number. The book said when comparing something, like 1 lb = 16 oz, it is an exact number because the two are the same (or exact). If I have 5 apples, it is an exact number. But it says if I have a measured quantity like temperature, or weight, it can vary according to measurement sometimes. It says for example if I measure a number with a ruler and it goes by units of 1, between 1 and 2, like 1.5. Both are significant numbers, yet according to the book (and since 5 is a non zero I think). The .5 in 1.5 can vary depending on measurements. Why is it then a significant number? If the .5 in the number is uncertain? From my book it says: Significant figures are all the digits including the estimated digit. Does this make any sense? Maybe i'm confusing significant numbers with measured or exact numbers, but, I really want to know what makes numbers significant.Not just that they are. I thought significant numbers had to do with uncertanity measureing them, versus an exact number. Thanks much for the help.