Just a few to keep your brains ticking over...

1. It happens once in a minute, twice in a week, and once in a year? What is it?
2. What goes up but never comes down?
3. Which is correct: 9 and 5 is 13 OR 9 and 5 are 13?
4. what trees come in two's?
5. Joan and Jane are sisters. Jean is Joan's daughter and 12 years younger than her aunt. Joan is twice as old as Jean. Four years ago, Joan was the same age as Jane is now, and Jane was twice as old as her niece. How old is Jean?