DHT11 is a commonly used sensor to detect the humidity and temperature in a lot of STEM project, it contains different advantage such as low cost, easy to use and digital output.
 
Working Principle
The DHT11 contain two major components to detect humidity and temperature respectively. The first component is using the Conductivity to measure the environment humidity. When humidity is changing, the polymer between two electrodes will change the conductivity, causing the resistance between electrodes drop or rise. The chip on DHT11 will use it to determine correct humidity. For the second component on DHT11, it is a Negative Temperature Coefficient(NTC) thermistor, when the temperature is rising, the resistance will decrease as follow. The chip will use that to determine the environment temperature.