system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: Why is 70 degree F water so much colder than 70 degree F air?

I can hang out buck naked in 70 degree F outside all day but not touching 70 degree F water.
gpt
It's because of the efficiency of heat transfer--when thermal energy moves from something warm to something cooler.  In this case, from your 98.7 degree (F) body to the 70 degree air or water.  Water is a better conductor than air, so when you get into the water heat escapes/is transferred out of your body at a much faster rate, leaving you feeling colder, faster than you would if you were just standing in 70 degree air.