Need a Central Idea for an Overall Science PYP Unit

Grades 3/4 are looking into starting a science afternoon every week, as we are a IBPYP school we were wondering if anyone had experience in writing a central idea that covers science in general.

We are looking to write a planner to cover a terms work in the different areas of Science (not a specific central idea for electricity for example).

Does anyone have any ideas or links that may help our 3/4 teachers?

Many Thanks