Answer:
Because sweat stains are mainly a mixture of minerals (especially salt) with the ingredients in antiperspirant or deodorant (primarily aluminum). When combined, they make an oily mess, making it harder to get rid of. Make a paste using four tablespoons of bicarb soda and a quarter of a cup of warm water. Rub this into the stain with your fingers and leave it out to dry for two hours. Then wash as usual. Should come right off.
Step-by-step explanation: