Fun isn't something that is given or done to you, it is something that you allow yourself to experience.
Back when we were children, this experience of pure delight didn't have to come from a heightened, heady event in order for us to feel like our day had been made; and it can be that way for us again - if we are willing to remember and reconnect with that part of ourselves that knows how to be in the flow of fun. You can begin this process by reminiscing on what was fun for you as a child. Think about what caused you to giggle in delight or wriggle in pleasure or burst into endless laughter that you couldn't sit up straight no matter how much you wanted to. Try to spend a few moments with each memory, and really feel what it was like to be in those experiences  allowing that feeling of pure fun to wash over you. It lives, in you  that feeling. It can't be bottled, manufactured, or sold. You just have to call it back up in order to experience it again.
Pure fun happens when we are fully engaged with ourselves and our world in each moment. It is the spontaneous delight that bubbles out of us when we let go long enough to bring it through; it is the experience of natural, organic pleasure that springs up from our bellies, through our souls, up through our faces, and down to our toes. We've naturally known how to have pure fun since we were babies and the flicker of lights caused us to jump to attention from the sheer enjoyment of being able to see. Approach your life today with the knowledge that pure fun isn't something that is given or done to you; rather, it is something that you allow yourself to experience.