What comes down but never goes up?

You do not want me to be permanent. But to avoid me is a mistake. You can let me help you. But precious time it will take.

Silky and soft we are, perfuming your lives. Take us to your love, but beware of our knives. What are we?

There are 30 people cruising on a boat in the Thames. However, when they emerge from sailing beneath London Bridge, not a single person is on the boat. How?

You can find me in the darkness, But never in the light. I make laughter lethal, And agreement into sight. You can find me in the soil, But never underground. A bunch of snakes together, Their voices do astound.

One falls but never breaks; the other breaks but never falls. They are opposites. They cannot coexist, but neither would know where they end or begin without the other.

Before I was taken I was used to take flight, and my partner's purpose was to help hide from sight. Brought together our function has been made anew, and now any scribe would be happy to have us in their retinue.