What is full of holes but still holds water?

I know a ten-letter word in the English language which can be typed using only the top rows of the computer keyboard.

I am a prime number.
The double of myself is equal to the square of me.

which number am I?

When I go in I might cause pain. I cause you to spit and ask you not to swallow. I can fill your hole. What am I?

In order to complete the racing competition, the Mexico racetrack has to submit its top and the most famous three horses to win the competition. Due to an electrical storm, all the records are cleared and no one knows which horse holds the record. They all look identical and it becomes even more difficult to differentiate the horses. There are 25 horses in the Mexico racetrack. But there can be only five horses at a time on the track. What will the least number of races that can be conducted to find out the three fastest horses?

Can you start the fire using two pieces of wood?

Replace the question mark with the correct number in below Picture:

I have cities, but no houses. I have mountains, but no trees. I have water, but no fish. What am I?

There is a barrel with no lid and some wine in it. 'This barrel of wine is more than half full,' said Curly. 'No it's not,' says Mo. 'It's less than half full.' Without any measuring implements and without removing any wine from the barrel, how can they easily determine who is correct?

What is in seasons, seconds, centuries and minutes but not in decades, years or days?