1. Odysseus chooses to sail toward Scylla rather than Charybdis because Charybdis would sink the whole ship and kill everyone
2. They are like mermaids, but they're scary and want to kill you. If they sing and you hear it, you will be put under a spell and lured to your death.
3. Scylla is a really nasty multi-headed and multi-handed sea monster. The danger is that it could eat all of them.
4. Charybdis is basically a whirlpool. The danger is that the ship could pulled inside.
hope this helps :)
