Which of the lines on the right, is the continuation of the black line on the left?

red one
blue one
none of them
both of them
This is known as Poggendorff illusion, first described in 1860. Many detailed studies of the illusion point to its principal cause: acute angles in the figure are seen by viewers as expanded.
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Very interesting!
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Answers by country

red one
blue one
none of them
both of them
answers
flagthe United States
47%
18%
20%
12%
272
flagthe United Kingdom
47%
16%
23%
12%
85
flagCanada
46%
20%
26%
6%
15
flagRussia
44%
33%
18%
3%
27
flagthe Philippines
43%
18%
25%
12%
16
flagPoland
40%
20%
22%
15%
11650
flagGermany
40%
24%
18%
16%
37
flagIndia
39%
29%
17%
12%
102
flagUnknown
38%
19%
28%
14%
21
flagUkraine
32%
23%
26%
17%
46
flagAustralia
15%
23%
30%
30%
13