Q: How do you stay warm in any unheated room?
A: Just sit in the corner where it is always 90º!
Explanation: To understand this joke, you need to think about the temperature. 90º Fahrenheit is a hot temperature. Well, for me it is hot; maybe not for people living in places like Australia, India, the Middle East and Phoenix that experience hotter temperatures.
Fahrenheit is a scale that measures temperature. Most of the world uses Celsius, not Fahrenheit, to measure temperatures (but this joke only works with Fahrenheit because 90º Fahrenheit equals about 32º Celsius).
90º is also the measure of a right angle, an angle that is found in a corner. This joke is funny because it plays with the two meanings of 90º (read ninety degrees)–a warm temperature and the measure of a corner.
Learn how to draw a 90º angle without a protractor-