i wanted to know how does starting fire with a water bottle really works Answered

It can work...
There are som things to consider though:
1. It is best to have a round bottle and by round I mean some part of the bottle should form a round shere - this gives the best focal point.
2. The bottle should be completely filled with water.
3. It works like any magnifying glass - you need enough sun or a big enough magnifying glass to start a fire.

A normal bottle can work too but you won't get a proper focal point, more a blurry line so intense sun is required.

Yes. It will work when there's a lot of sun.

A Magnifying Glass should work better though

Works best with a round flask borrowed from the science department!

Try this:

https://www.instructables.com/id/5-Ways-to-Start-a-...

Hint: Look through the full bottle. Now look through a pair of glasses or a magnifying glass. How are they similar?

Hint: It has to be sunny to work.