Imagine a bathtub. Water flows in through the tap. Water flows out through the drain.

The water level in the tub at any moment is a stock — an accumulation. It's the thing you can see, measure, and count right now.

The tap and the drain are flows — they change the stock over time. The tap is an inflow. The drain is an outflow.

This is the fundamental building block of every system: stocks accumulate. Flows change them.

Your bank balance is a stock. Deposits are inflows; withdrawals are outflows. Your reputation is a stock. Good work is an inflow; mistakes are an outflow. The CO₂ in the atmosphere is a stock. Emissions are inflows; natural absorption is an outflow.