A cat's normal body temperature is between 38 and 39 degrees Celcius (according to the vet) and a higher temperature necessitates a visit to the veterinarian. Here's how to take your cat's temperature reading in a safe way.
1) Hold your cat firmly (I held Blackie in a sitting down on all fours position) facing outward and lift its tail to expose the rectum. If using a mercury thermometer, remember to give it a quick shake with a flick of the wrist to "reset" the thermometer.
2) Insert the thermometer slowly and carefully into the rectum. Wait for 2 minutes if you are using a mercury thermometer (no wonder it didn't work initially as I only waited 30 seconds).
3) Remove the thermometer and read the temperature.