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On a dry and fine evening just after sunset, you have gone for buying a new shirt from a show room alongwith your friend down a lane. Lights are on in the streets and in the shops as well.

Due to some reason, your friend took few quick strides forward and he was ahead of you by 10 - 15 feet at which both of you happened to be infront of a shop/show room which has shirts displayed (needed by you) and also one of it, which is adjacent to the glass sheet and at around 6 - 7 feet away from you stole your heart and you decided to buy it.

At this moment, being sharp-eyed, you noticed a stain (or spot of dark color on a light back-ground) of considerable size and you are not sure where it is - on the shirt that was being displayed OR on the 1/2" thick crystal clear glass.

Here comes the actual question - without moving a single inch from your place, how can you decide if the stain/spot is on the shirt / on the glass sheet ?

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6 Answers 6

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Um, ask your friend to decide where the stain is for you.

But seriously

probably try closing one eye then the other to see the parallax effect. If the spot moves relative to the shirt then its on the glass, else it's on the shirt.

Alternatively,

Look for a reflection of yourself in the mirror where the stain is. If you can see one, then it's on the shirt else it's on the mirror

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  • $\begingroup$ Presumably, since you are a bit behind and your friend is a bit ahead, you should look for your friend instead of yourself. $\endgroup$
    – Jaap Scherphuis
    Commented Aug 29, 2017 at 11:16
  • $\begingroup$ Good one @Wen1now, but your spoiler content did not add much to the needed answer, though the first line did ! $\endgroup$
    – Mea Culpa Nay
    Commented Aug 29, 2017 at 11:20
  • $\begingroup$ If the shirt is lit by a light from outside the shop, and there are no other lights terribly near it, a stain on the glass could cast a stain-like shadow on the shirt; I don't think parallax would help, but if one could have one's friend move to block the light from that particular source, that would eliminate the stain-shaped shadow (if there was one). $\endgroup$
    – supercat
    Commented Aug 29, 2017 at 19:45
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The first thing that is come to my head. Of course a radical thing to do :

Throw a stone & break the glass.

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  • $\begingroup$ :-) A kind of a smoother solution please ! $\endgroup$
    – Mea Culpa Nay
    Commented Aug 29, 2017 at 11:18
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    $\begingroup$ But make sure it won't make another stain! ;-) $\endgroup$
    – CiaPan
    Commented Aug 29, 2017 at 12:23
  • $\begingroup$ Haha, doesn't matter if i tear the whole store apart; as soon as i can find out that the stain isn't on the shirt :)) $\endgroup$
    – Burak Mete
    Commented Aug 29, 2017 at 12:25
  • $\begingroup$ As a not-English speaker I presumably was not clear enough. I meant you may make sure the stain was on the glass ...and make a new stain on the shirt at the same time. Which would render the whole trick useless. $\endgroup$
    – CiaPan
    Commented Aug 29, 2017 at 13:28
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Have your friend look at the shirt and see if it has a stain.

If the stain is on the shirt, they will see it on the shirt as well. If the stain is on the glass, it will not appear on the shirt.
enter image description here

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  • $\begingroup$ Good, fully illustrated one @LeppyR64 ! $\endgroup$
    – Mea Culpa Nay
    Commented Aug 29, 2017 at 12:35
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You could

Ask your friend to move such that his reflection from the glass is at the same spot as the stain. If you can still see the stain then it is on the shirt, if not, it is on the glass.

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  • $\begingroup$ I'm not sure I follow how this would work... Surely whether the stain is on the glass or the shirt it will still be visible - a reflection won't obscure anything... $\endgroup$
    – Chris
    Commented Aug 29, 2017 at 14:16
  • $\begingroup$ if it is on the glass, it would get lighter/harder to see $\endgroup$
    – Dexter
    Commented Aug 29, 2017 at 14:21
  • $\begingroup$ Are you assuming that your friend is blocking some of the light to the glass? If the stain is on the glass and the illumination on it doesn't change then surely it will remain just as bright? If you are thinking about the contrast changing due to a different foreground reflection (ie it was reflecting a wall before, now it is reflecting your friend) then I would have thought that would effect the visibility of the stain equally no matter where it was... $\endgroup$
    – Chris
    Commented Aug 29, 2017 at 14:27
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I guess the wording is unintentional, but.

"Without moving a single inch from your place". You could just ask your friend to go and check and tell you, since he is free to move?.

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    $\begingroup$ And why would he need to move? $\endgroup$
    – user29705
    Commented Aug 29, 2017 at 15:17
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You could achieve the same effect as many other answers

By simply crouching down and standing back up and noting if there's any parallax

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  • $\begingroup$ Good one, @Conner F. Yes, as vertical motion is not restricted in my weaved puzzle ! $\endgroup$
    – Mea Culpa Nay
    Commented Aug 29, 2017 at 14:43

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