What occurs twice in a week, once in a year but never in a day?

Three fair coins are tossed in the air and they land with heads up. Can you calculate the chances that when they are tossed again, two coins will again land with heads up?

On a certain day, John celebrated his birthday. Two days later, his older twin brother Jacob celebrated his birthday.

Is this even possible? How can it be?

How can you make number one disappear by adding something to it?

What is the Surname of Barbie Doll?

What has no start, end, or centre?

Solve this riddle by finding the odd one out from the rest.

Things are just as they seem, right? Well, check again.
Can you count the number of tigers in this picture ?

Only one colour, but not one size,
Stuck at the bottom, yet easily flies.
Present in sun, but not in rain,
Doing no harm, and feeling no pain.
What is it?

An infinite number of mathematicians are standing behind a bar. The first asks the barman for half a pint of beer, the second for a quarter pint, the third an eighth, and so on. How many pints of beer will the barman need to fulfill all mathematicians' wishes?