Thanks to Michael* at Boots Opticians for suggesting this comic.  Apparently this is why none of the chairs in the shop have casters.

 

*I have a terrible memory for names.  I think he said Michael.

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2015-12-05 Optical Illusion, 5.0 out of 5 based on 2 ratings
↓ Transcript
Optometrist at tonometry machine: Now, you'll just feel a small puff of air.