We had a fun day at an outdoor play-land not far from where we live. They happened to have a giant slide made from what appeared to be white indoor/outdoor carpet of some sort that they hooked sprinklers up to and the kids would ride these sleds down it. It was a neat idea and it worked really well!
When I saw it I immediately thought of Newton's Laws at work and that this was a perfect moment to talk about science and do a little experiment! So I asked the three kids up front before they rode the slide "Which of you will go fastest down the slide do you think and why?" and once they each thought about it for a moment and gave me their thoughts/answer, I asked the next question "Do you think you will go faster or slower, if two of you ride together? What about three? Do you think it would go faster if the heaviest person was in the front or the back and why?" They gave answers, I asked more probing questions to guide their thinking and then, they made their predictions and they were excited to go off to try out all the various hypotheses they came up with! They first tried to ride down the slide individually to see who would go fastest. They discovered that the heaviest person went fastest.