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 won't be constant because the water pressure at the bottom depends on the height of the water in the bucket. Therefore, if the bucket is nearly empty, only a trickle will escape, but if the bucket is full, the extra weight of the water will push the water out of the hole much faster. Also, if the hole is bigger, the water will flow out much faster. For this model, we're going to assume that the outflow rate is proportional to the volume of water in the bucket multiplied by the size of the whole: Out_flow = Volume_In_bucket * Size_of_hole