Molearia

• Stock. A stock, visualized as a box, represents a storehouse. It accumulates and/or disperses something quantifiable as time passes. Examples are
  1. Money in a bank account.
  2. A population of rabbits.
  3. Water in a barrel.
  4. Drug concentration in the bloodstream.
• Flow. A flow, visualized as a pipe with a valve, represents a change. It increases or decreases the accumulation in a stock with each step in time. Examples are
  1. Monthly deposits to a bank account.
  2. Rabbit births per month; rabbit deaths.
  3. Water flow each minute from a faucet into the barrel.
  4. Injection of drug; elimination by kidneys.

• Converter.
  1. Monthly interest rate.
  2. Births per 100 rabbits per month.
  3. Opening/closing the faucet.
  4. Injection rate; elimination rate of kidneys.

• Connector.
  1. Interest rate affects interest deposits.
  2. Births per 100 rabbits per month affects total number of births per month.
  3. Opening/closing the faucet affects the flow into the barrel.
  4. The elimination rate affects how quickly the drug leaves the bloodstream.

Fill in a couple more stocks, flows, and converters of your own. Remember a flow can increase or decrease a stock. A Stella model must have at least one flow and one stock, but it need not have a connecter or converter. Think of a simple situation to model, then decide what is the stock and what is the flow. A stock should be measurable when time is stopped; a flow should be something that changes a stock over time.

Now we are going to build a model of a disease outbreak occurring at our school.
First we need some background information.

• There has been one confirmed case of molearia at your school.
• This disease causes an uncontrollable love of chemistry.
• The symptoms include balancing equations, trying to discover new elements, and a desire to write lab reports.
• Everyone at your school is susceptible to this particular disease.
• There are 1264 bodies at the school, and 73 students are absent.
• The infection rate for this disease is 0.005.
• The recovery rate is 1.0.
• Create a model to represent the susceptible, infected, and recovering groups of students.
• The Centers for Disease Control (CDC) has informed the school that the disease will be inactive 14 days from its onset.

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