timing how long it takes for your kettle to boil 1 cup (250ml) of water. looking at the power of your kettle (Watts, often found on the base of the kettle) joules = wattage x time (in seconds)
Rock climbing can help build and tone your legs and glutes. Indoor rock climbing is a sport that when properly executed, recruits every major muscle group in the body. Rock climbing is a full body workout that tones and strengthens the arms, shoulders and core muscles. You can also effectively strengthen and tone your legs and buttocks with regular indoor rock climbing sessions.
Rock climbing will certainly do more for upper body muscle than say, cycling. You will see some difference, but you will not really get beyond a novice level of strength. The most efficient way to improve strength is to dedicate training specifically to building strength (i.e. strength training).