Suppose we have a bucket sitting under a tap, with water flowing into it at a constant rate. The bucket has a hole in the bottom, so water leaks out over time. The rate at which water flows out of the hole will not be constant because the water pressure at the bottom depends on the height of the water in the bucket. Therefore if the bucket is full, the extra weight of the water will push the water out of the hole faster. Also if the hole is bigger the water will flow faster. For this model, we're going to assume that the outflow rate is proportional to the volume of the water in the bucket multiplied by the size of the hole.