Mary's lamb is a trap
25 Aug 2026
Earlier this week, I was looking for a simple way to illustrate to my teams why shared understanding and early discussions matter so much.
That’s when I remembered software pioneer Jerry Weinberg’s classic example from his book Exploring Requirements: Quality before design. He used the five-word nursery rhyme "Mary had a little lamb" to show how human language masks radical ambiguity.
When most people read that sentence, they immediately picture a young girl with a pet.
Cute, clear, and full of unstated assumptions. But depending on the unstated context, these alternative visual interpretations are equally valid.
Where you place the emphasis changes the operational focus entirely:
- Mary had a little lamb. (Identity: Was it Mary, or someone else?)
- Mary had a little lamb. (Temporal State: Does she still have it, or is this historical data?)
- Mary had a little lamb. (Cardinality: Exactly one, or at least one?)
- Mary had a little lamb. (Boundaries: Is size critical?)
- Mary had a little lamb. (Type: Was it a lamb, or actually a goat?)
The words themselves carry hidden semantic traps:
- Who is Mary? A human child, an adult ewe?
- What does "had" mean? Possessed, gave birth to, or previously owned?
- What is a "lamb"? A living entity, or a cooked meal on a plate?
If a simple five-word sentence can confuse living farm animals with dinner choices, imagine what happens when we hand an engineering team a single-line software requirement.
Frederick Vandenbosch
Quality Lead
He/him
Hi, I'm Frederick. I've been in software testing and quality since 2007, working as a software tester, quality coach, and more recently as quality lead. Continuously learning, experimenting, and solving problems are some of the things I enjoy most.
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Deborah Reid
I did a lightning talk at a conference about this last year! I used Old MacDonald as my example :D . It was inspired from a talk I saw by Emily O’Conner.
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