Why is a river so rich ?

I am looking for a word that change both gender and number when i add the letter 'S'.

What Word am I Looking For ?

There was a kingdom in which the king had no heir to take over his thrown. Even the queen was dead and he himself was on the verge of dying. He thought about it and then summoned all of the teenagers. He gave one seed each to all of them and asked them to grow the plant. He announced that the one with the most beautiful plant will become the king/queen of the empire after the death of the king.

After a month, all of them were called. The king looked at all of the plants but announced the girl with an empty pot as the queen of the empire. Why?

Find the Next Number in the Sequence

2 9 3 1 8 4 3 6 5 7 ?

Count the squares in the matchstick puzzle below?

Suppose we lay down two cups in front of you. One of the cups is filled with tea and the other one with coffee. Now we ask you to take a spoonful of tea and mix it with the coffee. At this moment, the coffee cup has a mixture of tea and coffee. You have to take that mixture (spoonful) and add it back to the tea.

Can you now tell if the cup of coffee has more tea or the cup of tea has more coffee?

An express train takes 3 seconds to enter the tunnel which is 1 km long.
If it is travelling at 120 km an hour, how long will it take to pass completely through the tunnel?

There are 2 people near the river and both of them want to get on the other side. The boat can only take one of them. But they got across.
How?

Which number comes next in the number series?

2, 9, 30, 93, ?

What is the word or phrase?