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

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?

Can you find a five letter word that is left with only two letters if one is removed?

After teaching his class all about Roman numerals (X = 10, IX=9 and so on) the teacher asked his class to draw a single continuous line and turn IX into 6. The teacher's only stipulation was that the pen could not be lifted from the paper until the line was complete.

A pond had 60 fish, out of which, all but 20 died.
How many fishes are now present in that pond?

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

A Car driver was heading down a street in Washington. He went right past a stop sign without stopping, he turned left where there was a 'no left turn' sign and he went the wrong way on a one-way street. Then he went on the right side of the road past a cop car. Still, he didn't break any traffic laws. Why not?

John named his first son Alpha, second Beta, third Charlie and fourth Delta. What is the fifth son's name?

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

Two girls were born to the same mother, on the same day, at the same time, in the same month and year and yet they're not twins. How can this be ?