Can you find out what is missing?

1 3 5
2 4 ?

Hint: It's not six. Think out of the box.

If an electric train is travelling south, then which way is the smoke going?

I have no doors but I have keys, I have no rooms but I have space, you can enter but you can’t leave! What am I?

What can run but never walks, has a mouth but never talks, has a head but never weeps, has a bed but never sleeps?

If nine thousand, nine hundred nine dollars is written as $9,909,
how should twelve thousand, twelve hundred twelve dollars be written?

One is to three as three is to five and five is to four and four is the magic number.
What is the pattern?

There is a hypothetical state between the USA and Mexico border 'Tango'.
Here 70 percent of the population have defective eyesight, 75 percent are hard of hearing, 80 percent have Nose trouble and 85 percent suffer from allergies, what percentage (at a minimum) suffer from all four ailments?

A four-digit number (not beginning with 0) can be represented by ABCD. There is one number such that ABCD=A^B*C^D, where A^B means A raised to the B power. Can you find it?

I am looking for a word that change both gender and number when i add the letter 'S'.

What Word am I Looking For ?

Different lights make me strange by changing me into different sizes. What am I?