The Second-Grade Store: Price, Substitutes, and Opportunity Cost
Mrs. Delgado's second-grade class runs a Friday store selling pencils and erasers to classmates. They buy pencils for 10 cents each and sell them for 25 cents. In October they sell about 40 pencils a week. In November, a teacher two doors down begins giving away free pencils from a box in her room. Pencil sales fall to 9 per week, while the class's erasers still sell steadily at 25 cents. The class treasury holds $18 and the April field trip needs $60. During a class meeting, Marcus says: "We should just charge a dollar. Then we'd make more money."
1. Evaluate Marcus's proposal. What reasoning does it get right, and what does it miss?
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What it gets right: he has grasped that revenue is price × quantity and that raising the price raises the first factor. That is genuine economic intuition and should be named as such.
What it misses: quantity is not fixed - it responds to price. At $1.00 for a pencil that is free two doors down, the quantity sold is likely to approach zero, and revenue with it. Comparing price alone, without quantity, is the error.
The second error is subtler: the relevant market is not "pencils in our store" but "pencils available to second graders this week," and that market now includes a free substitute. Raising price in a market with a free alternative is not a small miscalculation; it is the wrong direction entirely.
How to handle it with seven-year-olds: don't lecture - test. Have Marcus survey classmates: "Would you buy a pencil from us for 50 cents? For a dollar?" Run one week at 50 cents and compare units sold. Revenue = price × units sold is a two-line table they can fill in themselves.
Teacher note: the underlying concept is price elasticity of demand. Name it for your own planning; do not introduce the term.
2. Separate the economic concepts in this scenario: scarcity, shortage, and the effect of the free pencils. Which is which?
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Scarcity - permanent and universal: unlimited wants against limited resources. The class's $18 treasury and the $60 goal is scarcity in its clearest form. Scarcity never goes away; it is the reason choosing is necessary.
Shortage - a specific, temporary condition: the quantity demanded exceeds the quantity supplied at a given price. If the class runs out of erasers on Friday because they priced them too low, that is a shortage. It can be ended by a price change or a restock; scarcity cannot.
The free pencils - a substitute good: a free pencil in the classroom market drives the effective price of pencils toward zero. Students' demand for the class's pencils shifts left; they do not move along the same demand curve. Diagnosing it as a shift rather than a change in the class's own pricing is the key distinction, and it explains why eraser sales held steady: erasers had no free substitute.
Common student error to preempt: thinking the free pencil costs nothing. It costs the other teacher's time and the school's money - someone paid for it. Zero price is not zero cost.
3. Design a short activity that uses the $18 treasury to teach opportunity cost without the usual "it costs money" confusion.
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Setup: present two uses for the $18 - restock pencils or restock erasers - and force a choice. Each pencil nets 15 cents; fill in the per-unit numbers on the board.
The move that produces the concept: ask "If we spend the $18 on erasers, what do we give up?" The answer is not "$18" - the class still got erasers for it. The answer is the pencils they could have had. Opportunity cost is the value of the next-best alternative forgone.
Second move - the free pencil again: "The box of free pencils costs nothing. Is the cost zero?" This is where the concept sharpens: the cost is the teacher's time and the school's supply budget, and the opportunity cost to the class is the pencil business it displaced.
Exit ticket: "I chose ___ instead of ___. The thing I gave up was ___."
What to watch for: answers that restate the price paid ("it cost 10 cents") rather than the alternative forgone. That confusion is the target of the whole activity, and it is the one most likely to survive into middle school if it is not named now.