I guess they could have dropped him in and then he'd have been immortal - but that would have spoiled the subsequent story. Remember, it didn't really happen.
If I remember the legend, Achilles' mother dipped him in the River Styx because its water would ensure he was imune from normal weapons. She held onto his heel, because otherwise he would have fallen in and drowned! Of course, this proved fatal in later life, and did imortalise him with a saying for a vulnerable part of someone or something.
Cathy - surely that was Baldur, in Norse mythology, who was killed by mistletoe? Achilles was killed by an arrow which struck him on his heel - the only invulnerable part of his body.
Tis a fair point. Not only would Achilles have been completely invulnerable but his Mum's hand would have been too. Perhaps she'd just painted her nails?
Yes, it was because she held onto his heel whie dipping him in. I don't know why she didnt just dip her hand in aswell ... perhaps she was a witch and couldnt touch water.
Drinking fom the River Styx wiped out a man's memory, she was probably paranoid about getting it on her hands.
or needed a place to smack him where it would hurt if he was ever a naughty boy.