Why do we preferably have round manhole covers and not square ones?

Take one out and scratch my head, I am now black but once was red. What am I?

What has many keys but can’t open a single lock?

You can hear me, but you can't touch or see me.

What am I?

Assume the given figure to be a delicious doughnut. Yes, now you can concentrate more on the puzzle. So you have this delicious doughnut in your refrigerator when your friends come knocking at the door. There are eight of them. Now you have to make three cuts in this doughnut so that each one of you nine people can enjoy a piece of it. Neither you nor your friends would mind the size of their piece as long as they are getting it. How will you do it?

Can you place three balls such that the equation shown in the picture holds true?

15 caves are arranged in a circle at the temple of doom. One of these caves has the treasure of gems and wealth. Each day the treasure keepers can move the treasure to an adjacent cave or can keep it in the same cave. Every day two treasure seekers visit the place and have enough time to enter any two caves of their choice.

How do the treasure seekers ensure that they find the treasure in the minimum number of possible days?

Find the next number in the series.
1 10 24 43 67 ?

Which wheel of car does not rotate when you turn your car right?

What number do you get when you multiply all of the numbers on a telephone's number pad?