How you wondered why are these patches of blurry stains always found on the glass window of a public bus?

We can't deny that public bus is our daily means of transportation, it's not only for students but also those working adults.
This also means that those categories of people play a part in contributing such filthy stains on the glass window.
Have you noticed, especially during peak hours where people goes to work/ school and when people comes back from work/school, you will often see people leaning their heads on the glass window (it's much more comfortable to sleep this way) ?
I have conclude that those stains were caused by the oil-iness of someone's hair / face !

Imagine you leaning your cheek on it ? Gross !
And if you ever tried to clean it away with a tissue, I can assure you that those stains won't come off.

Story Time:

During my secondary school days till now, I never fails to sleep on the bus during bus journeys. And the best way to sleep on the bus is to lean ur cheek on the glass window beside you, agree ?
So during one of my trip to school on a double-decker bus, as usual, I will just sleep on the bus. Initially, I did not realise that there was a stain on the glass window beside me. When I was awakened by the the jamming brake of the bus,I looked out to check if everything was alright.
With astonishment, I actually saw the stain on the exact spot where I rest my head on. Wondering if i have contributed to that, I swiftly clean that way with a tissue, hoping that no one sees me, but the stubborn stain just wouldn't come off.
I've convinced myself that it must be the previous passengers who left the stain, not me ! It was still an embarrassed moment for me but a new DISCOVERY !