Teacher gets on one end of the see-saw. Have one student try to push the other end down and lift her up. Keep adding students. See how many students are needed and approximately how high the teacher is lifted off the ground. Move the fulcrum of the see-saw and repeat the process of adding students until the teacher is lifted.
All levers have three parts.
A see-saw is just one type of lever. The bucket should be heavy and this should be pretty difficult for the students. I have a couple of wheelbarrows here and we can take turns moving the same buckets the same distance and see what happens. Return inside and have the students pick one of the two types of levers, describe it and name its parts. Check the closure activity.