We've all done this: Decide you need to relax with a soak in the tub. Start filling the tub and walk away to get all the accouterments prepped for your relaxing bath (crossword, music, candles, bath oil, etc.). Excitedly come back to a tub, nice and full. Get in. Find out it's scalding hot. Jump out of tub screaming. Look at your foot to see whether it is still there or burnt off entirely.
Avoid this awful situation by getting a bath thermometer. Commonly used by moms who want to make sure their baby's baths are at the appropriate temperature, these bath thermometers can be used by adults who are easily distracted too! The ideal bath temperature, so that you don't shock your body, is about 98-99 degrees (though some like it hotter). However, usually if it's any hotter, you'll be sweating to cool down, and if it's any colder your body will be in overdrive trying to warm itself up — hardly the relaxing experience you were looking for.
I especially like this Mobi Digital Bath and Room Thermometer ($12.99) because it also measures the room temp as well, which is kind of cool. Buy it from target.com.