If I remove one from eleven it becomes Ten and if I remove one from nine also becomes Ten. How?

Without looking at a calendar, within a minute name, a boy's name using 5 consecutive 1st letters of 5 consecutive months

The more you take me, the more you leave me behind.
Who Am I?

I am a type of vehicle that has two wheels and is powered by humans. What am I?
Hint 1: You can ride me for exercise.
Hint 2: I am environment-friendly.

I came first on earth but second on the heaven.
I also came twice in a week but found just once in a year.
I stay away from months but you can find me in February.

What am I?

How many sides does a circle have?

I sleep during the day and fly at night, but I have no feathers to aid my flight. What am I?

How can you write nineteen in a manner that if we take out one, it becomes twenty?

If six children and two dogs were under an umbrella, how come none of them got wet?

If eleven plus two equals one, what does nine plus five equal?