How can I make things float ?

I have a science project/competition which is to "Make Things Float" and if I win then me and my class get to go to Flamingo Land... Anyway, does anyone know how I could achieve this ? I know you can make an egg float in water (not at the bottom or top of a tank of water, IN THE MIDDLE) however I can't remember how to do that ! Please state any other suggestions below..x Thank You For Your Time.