Exercise 2. 'Water in bathtub' is the stock, measured as a volume of water, in L. The bath initally holds 100 L of water. The bath receives water from the taps ('Variable inflow from tap', in L/min) and loses water to the drain ('Constant outflow to drain', in L/min). 'Twiddling the taps' gradually ramps the inflow between a minumum of 0 L/min and a maximum of 60 L/min within each 5 minute (300 second) intervals. The outflow remains constant at 30 L/min.

Sales initially grow as the new product moves from introduction into growth.  Sales success attracts competitors who introduce their own products which reduces sales growth of the new product.

Eventually the total number of products that the market can bear are sold which stops further sales.

More advanced models could contemplate how competition affects price, how new successful products from competitors take market share, etc.