A few nights ago my son and I enjoyed playing around with these water buckets in the bath tub. Simply watching the water stream out of the little holes at the bottom of the bucket is entertaining. As the water level inside the bucket decreases, the speed of the water coming out of the holes, and the distance it travels decrease. The proportional relationship requires some real algebra skills to quantitatively describe, but we could see it pretty easily. The less gravity can push down on the water in the bucket, the less the water coming out of the holes is accelerated.
However, the water is a fluid, which means that it is affected by pressure. Pressure increases when force increases and decreases when effected area increases. The bucket will fill only until the inner and outer pressures are equal. Since the inner surface area is smaller, the same pressure can be reached with less water.
So what about those demos that show the bottled filled with water above the water line? We didn't play with those, so I will have to save it for another post. I will say, that the different results demonstrate why I think it is really important that we spend time playing around with the typical every day world. We need to fully appreciate the every day events before we can really enjoy the special cases--and that's exactly what we did.