Suppose their is a bucket with water inside it and there is a small hole at the bottom of the bucket such that water leaks from the end. Area of cross section of the bucket is A and area of the small hole is a. The velocity with which water is coming out of the hole is v and the velocity with which water is coming before coming out of the hole is dh/dt, where h is the height of water inside the bucket and h is decreasing with time which is obvious. Now, if we apply equation of continuity , we get A(-dh/dt) = av. My question is, why do we include the minus sign in LHS? Is it because dh/dt by itself is negative and we need to make it positive? But why do we need to make it positive in the first place?