Can you replace the question mark with the correct number?

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 man stands on one side of a river, his dog on the other. The man calls his dog, who immediately crosses the river without getting wet and without using a bridge or a boat. How did the dog do it?

Prove that twice of ten is the same as twice of eleven.

By removing one matchstick and adding one matchstick, can you convert the word "ZOO" to an animal name?

Move three sticks of the spiral below to form a square.

Analyze and find the missing item in the sequence:
16, 06, 68, 88, __, 98

What came first, the chicken or the egg?

Two guards were on duty outside a barracks. One faced up the road to watch for anyone approaching from the North. The other looked down the road to see if anyone approached from the South. Suddenly one of them said to the other, "Why are you smiling?"

How did he know his companion was smiling?

Luis Garavito killed Density x Volume.

Who is Luis Garavito ?