En fact i love foot sweat,when feet are unwashed and used all the day! If the shoes are old, the insoles is marked with the foot print. It help to make gross dirt. I think when you have hardskin and you didn't do pedicure some of the deadskin peel off from the foot and if you sweat its mix with it! Its usualy called " TOEJAM". Thats what i love! Its a kind of pungent stinky sweaty feet... Its make by the skin... You could check this! Its exactly what i love!!!!