Looking on the internet deeply has found these results:

there is a fly on the window is the most popular phrase on the web.

there is a fly in the window 11 results on the web	there is a fly on the window 15 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
Nobody knew till they saw the flies against the window. In truth, there was a fly on the window. Okay, then fly, fly out the window. I let rodrigo fly out the window. Let me also point out that there is a fly in the ointment. But if you end the group now, all my hard work flies out the window and you're left with a bunch of angry prisoners. There is a fly in the ointment, however. When you're poor, love flies out the window.	In truth, there was a fly on the window. You know what else flies out the window? Nobody knew till they saw the flies against the window. When you're poor, love flies out the window. But if you end the group now, all my hard work flies out the window and you're left with a bunch of angry prisoners. I let rodrigo fly out the window. Okay, then fly, fly out the window. One has only to mention the woman's name and decency flies out the window.

there is a fly in the window

11 results on the web

there is a fly on the window

15 results on the web

More popular!

Nobody knew till they saw the flies against the window.
In truth, there was a fly on the window.
Okay, then fly, fly out the window.
I let rodrigo fly out the window.
Let me also point out that there is a fly in the ointment.
But if you end the group now, all my hard work flies out the window and you're left with a bunch of angry prisoners.
There is a fly in the ointment, however.
When you're poor, love flies out the window.

In truth, there was a fly on the window.
You know what else flies out the window?
Nobody knew till they saw the flies against the window.
When you're poor, love flies out the window.
But if you end the group now, all my hard work flies out the window and you're left with a bunch of angry prisoners.
I let rodrigo fly out the window.
Okay, then fly, fly out the window.
One has only to mention the woman's name and decency flies out the window.