Science Puzzle
The Happiest Plants in the Lab
A student proposes the hypothesis: "Plants grow better when they feel happy." She is convinced it is true and wants to test it.
Her supervisor tells her the hypothesis cannot be tested as written, and that this is a problem with the hypothesis rather than with her equipment. What is the problem, and what would fix it?
The Answer
The hypothesis contains a term nobody has defined in measurable form. "Happy" has no operational definition: no stated procedure that would produce a number, a yes, or a no. So no observation could ever settle it, which means the hypothesis makes no prediction and forbids nothing.
An operational definition is the bridge between an idea and an experiment. It replaces a concept with the specific procedure you will use to measure it. Not "happy", but "exposed to 60 decibels of music". Not "grows better", but "increase in stem height in centimetres over thirty days". Once those are fixed, the hypothesis becomes a genuine prediction that a set of measurements can contradict.
Notice what this does and does not do. It does not settle whether plants have feelings, and it is not a claim that the concept of happiness is meaningless. It simply says that a term with no agreed measurement procedure cannot appear in a testable claim, because there is no fact of the matter about whether the experiment came out for or against it.
The second option in the list is a trap worth naming: "plants obviously cannot feel anything" rejects the hypothesis on the grounds of being false rather than untestable. That is a different objection, and a weaker one, because it settles the question by assumption before any evidence is gathered. The methodological point is sharper: even if plants did have feelings, the hypothesis as written would still be untestable, because you have not said how you would tell.
This is one of the most useful habits in the whole method. When a claim resists testing, the fault is very often a single undefined word sitting in the middle of it. Find that word, ask "what exactly would I measure, and what reading would count as which answer", and either the claim becomes testable or it quietly collapses.
The principle: Operational definitions. A hypothesis is testable only when its terms are replaced by specified measurement procedures. An undefined term makes a claim unfalsifiable regardless of whether it is true.