Here's a little puzzle that may provide some amusement. (I hope it's not too trivial!) I have attached a (rough) diagram to accompany the following description. Consider an empty tank with a circular hole in its bottom. (The tank is supported by a table that also has a hole it.) Also consider a steel cylinder that just perfectly fits within the hole, making a frictionless, watertight seal. The cylinder (which has mass m, area A, length L) is suspended by a thin, massless cord attached to the ceiling. The cylinder extends half-way through the hole in the (empty) tank. The tension in the rope is equal to mg, the weight of the cylinder. Question: What happens to the tension in the cord as water is slowly added to the tank?