This is a fantastic analogy I refer people to when they wish to understand my limitations from ME/CFS. It's a story written by Christine Miserandino called 'The Spoon Theory', based on her experience of explaining what it's like to live with Lupus to a close friend. Granted, some of us may have less 'spoons', smaller 'spoons', or 'spoons' of different shapes, but the analogy still applies to everyone with ME/CFS and really gets the point across!
http://www.butyoudontlooksick.com/navigation/BYDLS-TheSpoonTheory.pdf