Why Aeroplane windows are round?

In the figure, you can see nine stars. What you have to do is connect all of them by using just four line and without lifting your hand i.e. in a continuous flow.

Why is a river so rich ?

Out of the nine cats that you can see in the picture, there are only two that are completely identical. Can you find them out?

Four friends are sitting on a tree. Each one of them is wearing a hat. They don't know which coloured hat are they wearing but they do know that two of them have green hats and two of them have orange hats.

Adam is sitting on the top branch and he can see the hats of Billy and Cyrus. Billy is sitting on a branch above Cyrus and he can see Cyrus hat. Duke is sitting at the base of the tree. Branches are covering him fully and no one can see his hat.

Now they are not allowed to look upwards. Who do you think will shout the colour of his hat first?

Can you replace the X (both small and big x) with the numbers between 1 to 19 such that all the rows of 3 numbers between X (big x) sum to 23?

Which of the following words don't belong in the group and why?
CORSET, COSTER, SECTOR, ESCORT, COURTS

Can you convert the number zero(0) into a positive number using just one mathematical operator ?

Take away my first letter, then take away my second letter. Then take away the rest of my letters, yet I remain the same. What am I?

How many letters are in the alphabet?