What is the correct way to insert a thermometer into the rectum?
Taking a rectal temperature is exactly as simple (in theory) as you might imagine.
You insert the mercury side of the thermometer (or on a digital thermometer, the skinny side that does not show the numbers) into the anus about a half an inch in (just far enough to cover the tip of the thermometer). Leave it in about 45 seconds (or, again, if using a digital thermometer, until it beeps). Some people recommend lubricating the end to make it more comfortable, but honestly, a half an inch is not far at all and in my experience there is little need for lubrication.
Typically, taking the rectal temperature is used on babies and small children, as this is the most reliable reading you can get with any accuracy. A normal rectal temperature of a child is between 97 and 100 degrees.
That said, babies don't really like it. Depending on the size of the child, you'll want to position him on his stomach and I think it works best to have the knees tucked under like a yoga "child's pose." Once you've inserted the thermometer, keep it balanced in the crook of your fingers and hold the babies entire bottom in your hand, gently holding the cheeks closed... this will keep it stable for the minute it takes to get the reading.
Of course, be sure to wash and disinfect the thermoter after use.