What is the next number in this series?

4,12,84,3612....

What does this rebus mean?

O
N
C
E
TIME

Why do we preferably have round manhole covers and not square ones?

Which part of London exists in France as well ?

There are three light switches outside a room. One of the switches is connected to a light bulb inside the room.
Each of the three switches can be either 'ON' or 'OFF'.

You are allowed to set each switch the way you want it and then enter the room(note: you can enter the room only once)

Your task is to then determine which switch controls the bulb?

What does this mean?

CCCCCCC

What is wrong in the below Picture?

By including only four matchsticks in below picture, you have to make "Four Triangles And Two Squares"

Find the hidden panda in the image below.

I have a five and a three-gallon jar. Assuming there is an infinite supply of water, how can I measure one-gallon water?