The hall is silent. The only sounds your over-strained ears can pick up are the occasional rustling of paper and the scratching of the pen (yes, the phrase is borrowed from the Order of the Phoenix replaced with pen instead of quill). The physics examination was in session.
A student walks out an hour early.
Later, he is summoned to the office by his teacher to reprimand him for his abysmal performance.
Teacher: You need to answer this question correctly to pass, I'm giving you a last chance
Student: But what's wrong with my answer?
Teacher: Read this.
Q: How do you measure the height of the building using a barometer?
A: Tie a string to the barometer and let it down until the barometer just touches the ground. Add the length of the string to the length of the barometer to get the answer.
Teacher: How much do you expect for this answer?
Teacher: I'm giving you five minutes to tell me the right answer. Otherwise, you fail.
Student looks aimlessly for three minutes.
Teacher: Well, shall I fail you? Don't waste my time, do you know the answer?
Student: Well sir, I have many answers, I didn't know which to choose so I'll mention all of them.
You could measure the length of the upright barometer's shadow and the length of the building's shadow. By using the known length of the barometer, with a simple ratio you could ascertain the building's height.
You could tie a string to the barometer and use it like a pendulum. By calculating the acceleration due to gravity at both the ground level and at the top of the building and use the minute difference to measure the height.
Or, you could measure the air pressure at ground level and air pressure at the roof in millibars and substitute in this formula to find the height.
Personally, I'd just ask the janitor.
The student passed.
His name was Niels Bohr.