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

A Scientist was fifty years old in the year 2000 but he was forty years old in the year 2010. How can this be possible?

PS: It has a logical answer.

Look at this sequence from top to bottom. What is the next number in the sequence?
1
11
21
1211
111221
312211

You need to arrange three 7 and mathematical symbols to form number 1?

A network of 20 x 10 squares is given to you.

Can you calculate how many unique squares and rectangles can be formed combining two or more individual squares ?

Why is Saturday stronger than Monday?

I have a clock(12-hour format) and both the needles of the clock overlap at 12:00.
After how much time, they will overlap again?

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?

Rectify the following equality 101 - 102 = 1 by moving just one digit.

What's the next in the series?

H, Be, F, S, Mn, Kr, In, Gd, Tl, ?