After the lab session, you will have a record of what you did in the lab, and any data that you collected. The experimental script may have questions for you to think about, or there may be further work required in order to process the data, perhaps to calculate a parameter or reach a conclusion. Your lab book is a sensible place to record this material, so that all of the information about the experiment (preparation, plans, what actually happened in the lab session, data, calculations, conclusions etc.) is in the same place. For some practical courses, you may be required to complete a report for submission and assessment; your lab book entries will be the main resource you rely on to complete this work.