Traditionally, the "Climate Bathtub" model includes a single bathtub which represents Earth's atmosphere. The flow into the tub from the faucet represents carbon dioxide being added to the atmosphere. The flow out of the drain represents carbon dioxide being removed from the atmosphere.
But where does the "water" pouring out of the faucet come from? Where is the water flowing out of the drain going to? The bathtub model represents just a portion of the larger carbon cycle. We can expand upon the bathtub metaphor by adding a cuple more tubs representing major "reservoirs" in the carbon cycle. The animation below indludes three bathtubs, representing the atmosphere (top tub), the oceans (lower right tub) and plants (lower left tub).
This animation represents an equilibrium state of the carbon cycle: the water levels in the tubs remain steady. Such was the case at many (but not all!) times during Earth's history prior to the increased human-caused CO2 emissions of the last couple of centuries. This model is, of course, just an approximation; water would not flow uphill to the top tub unless there were pumps along with the pipes to overcome the downward pull of gravity. This version of the bathtub model still only represents a portion of the full carbon cycle; we would need to add several more tubs and lots of pipes, faucets, and drains to create a more complete depiction. However, this three-tub model may serve as an adequate bridge to help students make the conceptual leap from the somewhat concrete bathtub model portrayal to more asbstract carbon cycle diagrams.