By this age, your child will be quite familiar with a playground. By observing a see-saw, she can now easily tell the 'heavier' and 'lighter' side of it. When riding a see-saw, your kiddo will understand instinctively by its movement if she is able to ride it properly or not depending on her partners weight.
What you need to know:
At this point, your kiddo will realize that if a see-saw goes down on the other side and does not balance on her other side easily, then her partner is heavier than her. Thus this understanding helps her to choose her partner. As a parent you can introduce your child to the physics behind the working of a see-saw in simple words, so that in the coming years in her science lessons she would be easily able to relate to this concept. It won't be a new concept for her. You can introduce her to words like 'lever', 'hinge', 'force', 'gravitational force' etc. and illustrate similar examples in daily life. This helps in learning and better understanding.