The clinical thermometer should not put in hot water because it may burst and injure the user or the person nearby
The idea is based avoiding use of the thermometer when it is reading "higher" than what it is that is being measured. Let's look at what's happening. If we wash a clinical thermometer in hot water, it will respond to that hot water. That will cause the reading to shoot up well above a hundred degrees. If we then try to use it, it will have to "cool down" under the tongue of the person we're using it on. It is actually being cooled by that person. The thermometer will take a relatively long time to reach the temperature of the individual whose temperature is being measured. And this time will vary, but will be longer than it would be for the thermometer to "come up" to temperature. One of the directions for using these thermometers is to first shake it down until it reads less than 95 degrees or so. If you're starting with a thermometer than has just been washed in hot water, you're not going to be able to shake it down per the directions. You'll have to run it under cool water for a bit to cool it so you can use it.