In elementary differential equations, there's one particular example that is as educational as it is simple: a leaking tank of water. In its barest form, this is just a tin can, open at the top, with draining out of a hole at the bottom. It has a much deeper side, though, because it models a whole host of familiar real-world phenomena. For example, water can be heat flowing out of a cooling cup of coffee; or it can be electric charge draining from a capacitor; or even carbon C-14 leaking (decaying) to normal C-12, familiar in radioactive dating. The leaking tank is obligingly versatile, too. For example, one can run a movie of it backwards; this corresponds to exponential growth-from population growth to increasing principal in a savings account. Or one can take an empty tank and push it part way, bottom first, into a large body of such as a lake, causing to flow into the tank through the hole; this models, for instance, an abandoned cold soda warming up to room temperature, or an ocean liner reaching cruise speed. If the submerged tank's level starts higher than the lake's, it mimics a parachutist slowing down to terminal speed. With the right assumptions, the differential equation governing a leaking tank directly reflects the physical setup. To this end, we begin by supposing that the tank is cylindrical with base area 1; this allows us to track the tank's volume using the height y(t). Next, this height will always approach a steady-state level, and for simplicity we choose the origin of the y-axis to be this level. Finally, if k denotes the cross-sectional area of its drain, we choose units so that the level y satisfies y'= -ky; this equation and the physical setup are now direct translations of each other: at any time, the flow rate equals drain area times height. The solution of this equation is y(t) = AOe-kt, where Ao is the initial level. Notice that the larger k is, the faster the approaches its steady-state level. Interestingly, the physical dimensions of a tank translate to, and even claiify, various physical notions. As just two examples, the thermal equivalents of volume, height, tank base area, and drain area are: heat, temperature, specific heat, and thermal conductivity. Electrical counterparts are: charge, voltage, capacitance, and electric conductance (the reciprocal of resistance). Learning to translate to and from tank-lish helps one unify parts of the physical world; one can profitably get hooked on it.