Easy riddles

There was once a college that offered a class on probability applied to the real world. The class was relatively easy, but there was a catch. There were no homework assignments or tests, but there was a final exam that would have only one question on it. When everyone received the test it was a blank sheet of paper with a solitary question on it: "What is risk?" Most students were able to pass, but only one student received 100% for the class! Even stranger was that he only wrote down one word! What did he write?

What bone will a dog never eat?

What makes grown men cry, but humanity would go extinct without it?

What do monkeys sing at Christmas?

How do you make the number one disappear?

What jumps when it walks and sits when it stands?

What did the paper clip say to the magnet?

I have seas without water. I have forests without wood. I have deserts without sand. I have houses with no brick. What am I?

Wash it and it isn't clean. Don't wash it and then it's clean. What I Am?

Four people need to cross a rickety bridge at night. Unfortunately, they have only one torch and the bridge is too dangerous to cross without one. The bridge is only strong enough to support two people at a time. Not all people take the same time to cross the bridge. Times for each person: 1 min, 2 mins, 7 mins and 10 mins. What is the shortest time needed for all four of them to cross the bridge?