Two real investigations, one virtual experiment, and 37 points. Plan on about 32 minutes.
Click any card to read its meaning. Only one card stays open at a time, and each card closes on its own after 8 seconds. You can reopen a card as many times as you want.
Cards never lock. Reopen anything you want to review before you match.
Click a term on the left, then click the meaning that goes with it on the right.
In 1904, crowds packed a Berlin courtyard to watch a horse do arithmetic. His name was Hans, and his owner was a retired math teacher named Wilhelm von Osten. Von Osten would ask a question out loud — what is four plus three? — and Hans would tap his hoof seven times and stop. The horse answered questions about dates, musical notes, and spelling. He was right almost every time. Newspapers called him Clever Hans.
Von Osten charged no money and seemed completely honest. A panel of thirteen experts, including a circus trainer and a veterinarian, watched Hans work and reported that they found no trick. That report convinced most of Berlin. It did not convince a young psychologist named Oskar Pfungst. Finding no trick is not the same as showing there is none.
Pfungst realized there were two possible explanations, and the crowd had only ever considered one. Either Hans understood the questions, or Hans was picking up some other signal that happened to arrive at the right moment. A good test had to be able to tell those two apart. Watching more closely would never do it. Pfungst needed to change something.
He started by letting strangers ask the questions instead of von Osten. Hans kept answering correctly, which ruled out the idea that his owner was secretly cueing him. Then Pfungst tried the test that mattered. In some trials the questioner was told the answer before asking. In other trials the questioner was handed a sealed question he had never seen, so he did not know the answer either. Everything else stayed the same: the same courtyard, the same horse, the same kinds of questions, the same crowd.
The results split cleanly. When the questioner knew the answer, Hans was right almost every time. When the questioner did not know the answer, Hans was almost always wrong. Pfungst then fitted Hans with blinders so the horse could not see the person asking. Hans failed again, even when the questioner knew the answer.
Hans was not doing math. He was reading tiny shifts in the questioner’s posture and face — shifts so small that the questioner did not know he was making them. As the tapping approached the right number, the person tensed slightly, and Hans stopped. The graph below shows how Hans performed across four test conditions.
Percent of questions Hans answered correctly under each of Pfungst’s four test conditions.
1. What was the independent variable in Pfungst’s key test?
2. Pfungst kept the courtyard, the horse, and the kinds of questions the same in every trial. Why does that matter?
3. Which claim is best supported by the graph?
4. Pfungst proposed a stating that Hans was responding to a signal from the questioner instead of doing math.
5. Trials in which the questioner did not know the answer worked as a because nobody in the courtyard knew the right number of taps, so nobody could signal it.
6. Because Hans scored well only when the questioner knew the answer, Pfungst had that supported his idea.
Build the sentence in English. Words in the right spot turn green.
Pfungst cracked the Clever Hans puzzle by changing one thing and holding everything else steady. Now run a test of your own. You press your fingertips onto a nutrient plate after scrubbing for a set number of seconds, then count the bacterial colonies that grow after 48 hours. Everything else stays the same every time: same soap, same water temperature, same plate, same 48 hours at 37°C.
Run all five scrub times. Each result is recorded automatically in your data table.
| Scrub time (s) | Colonies counted |
|---|---|
| 0 | — |
| 5 | — |
| 10 | — |
| 20 | — |
| 30 | — |
7. What is the independent variable in this simulation?
8. What is the dependent variable?
9. Which of these is a controlled variable in this simulation?
A data table stores numbers. A graph shows the pattern hiding inside them. Build the graph from your own five trials, then look for the trend before you answer.
10. Which statement describes the pattern in your graph?
11. Between which two scrub times does the colony count drop the most?
12. You ran each scrub time only once. Why would a scientist run each one many more times?
Scientists share their work in papers that all follow the same shape: what they asked, what they did, what they found, and what it means. Once you know the shape, you can read any of them. Here is one, written the way a real journal would print it.
Hand cleaning is recommended to reduce the spread of illness in schools, but students disagree about whether rinsing with water is enough. We compared bacterial colony growth on fingertip plates after three cleaning methods.
Sixty seventh-grade volunteers from one school were randomly sorted into three groups of twenty. Group A did not clean their hands. Group B rinsed with plain water for twenty seconds. Group C washed with liquid soap and water for twenty seconds. Every participant then pressed the same four fingertips onto a nutrient agar plate. All plates were sealed and held at 37°C for 48 hours. A researcher who did not know which group each plate came from counted the colonies.
| Group | Cleaning method | Mean colonies per plate |
|---|---|---|
| A | No cleaning | 84 |
| B | Water only, 20 s | 51 |
| C | Soap and water, 20 s | 12 |
Washing with soap and water for twenty seconds left far fewer colonies than rinsing with water alone. Rinsing with water was better than doing nothing, but it removed less than half of what soap removed.
All participants attended one school, and only one brand of liquid soap was tested. Colony counts measure bacteria present on the skin surface, not whether a student later became ill.
13. Which group served as the control group?
14. Which conclusion is supported by the results table?
15. The researcher who counted colonies did not know which group each plate came from. Why does that matter?
16. Before a journal printed this study, other scientists checked the methods and the math through .
17. The number of colonies counted on each plate is the in this study, because it responds to the cleaning method.
18. Stretch this bare sentence: Pfungst tested an idea.
Add when and where, then add why or how. Write one full sentence of at least 12 words.
Five questions drawn from a bank of twenty. You need 3 out of 5 to reach mastery. If you fall short, you can try again with a fresh set of questions.
Printing creates a PDF with your ID number, your grade, your data table, both graphs, and every question with the answer you gave.