This question may be well known to some, but I still think it is interesting:
It comes from the 1964 Louisiana literacy test which you had to take to be eligible to vote.
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Are there people who don't know this? :)
Paris in the the spring
The word 'the' is repeated; people often miss it, as the way we read tends to gloss over unimportant words.
The alternative answer, for the pedantic grammarians, is
what you read in the triangle below.
but that assumes you insist on ignoring the clear surface intent of the question. Both alternatives, honestly, qualify as "an annoying way to trick people". The alternative just amps it up to 11.
The illusion is that when you first read it you see
Paris in the spring
But it actually reads
Paris in the the spring - two 'the's