There is no such thing as a perfect measurement. Each measurement contains a degree of uncertainty due to the limits of instruments and the people using them. In laboratory exercises, students are expected to follow the same procedure that scientists follow when they make measurements. Each measurement should be reported with some digits that are certain plus one digit with a value that has been estimated. For example, if a student is reading the level of water in a graduated cylinder that has lines to mark each milliliter of water, then he or she should report the volume of the water to the tenth place (ie. 18.5 ml.) This would show that the 18 mls are certain and the student estimated the final digit because the water level was about half way between the 18 and 19 mark.