What is the easiest and safest way to calibrate a thermometer?

Over time, thermometers can lose their accuracy, and they should be calibrated regularly. And any time the thermometer is dropped or jostled, it should be calibrated again. "Calibrating" means checking the thermometer for accuracy and adjusting it if needed. The two most common ways to calibrate a thermometer are the "ice-point method" and the "boiling-point method".
The ice-point method is the easiest and safest because it does not involve boiling water and also because water actually boils at different temperatures depending on your elevation. To calibrate a thermometer with the ice-point method, fill a large glass or jar with ice and then add water. Crushed ice works the best. Take the temperature of the ice water. If the thermometer does not read 32°F, adjust it so that it does.
To calibrate a thermometer using the boiling point method, you must first determine what the boiling point of water is at your location. Usually, this is about 212°F. But if you live near the mountains, it may be a bit lower and if you live near the ocean, it may be a bit higher. Use Google to search for the boiling point in your city. Once you know the boiling temperature in your area, simply bring a pot of water to a rapid boil. Check to see if the thermometer gives the correct temperature and if it doesn't, then calibrate it so that it does.