Answer:
I am used when you jump off a bridge for fun.
The Titanic is still here. What am I?
I'm written with two letters, said with one and seen with two. What am I?
If you drop me I'm sure to crack but give me a smile and I'll always smile back. What am I?
You may enter, but you may not come in. I have space, but no room, I have keys, but open no lock. What am I?
Though not a plant, has leaves. Though not a beast, has spine. Though many wouldn't need this thing, It's more valuable than wine.