What you Need
What to do
You stand one side of the window and your friend the other, each ot you hold a torch.
Turn off all the lights so it is dark on both sides
Turn on your torch, what do you see in the window?
Turn off your torch and turn on theirs, what do you see?
Try turning on both.
What may happen
You should be able to see yourself when you turn your torch on, them when they have their torch on, and mostly them when you both have a torch on.
Why does it happen?
This experiment works because whilst glass is very transparent it will also reflect back some light. So probably 95% of the light goes straight through and maybe 5% reflects back. You don't really notice this when both sides of the glass are bright as the light going straight through is so much brighter than the reflection.
But if the far side is dark then there will be no light coming from there, so the only thing you can see is the reflection. Similarly if your side is dark then there is no light hitting the glass to form the reflection, so you can't see it.
This is why you can't see out of windows at night, because it is brighter inside or into them during the day, as then it is brighter outside.