Treading water

Who knew that treading water was such a good calorie burner and toner. At my current weight I can get rid of about 400 calories in a 30 minute span, and other than the tedium factor, it doesn't seem difficult at all to do. It's also good for all the major muscle groups.  I can feel it a little bit in my arms and core, but no soreness.  It can also be sort of contemplative and I find myself able to zone out during the treading.