PUZZLE THEM THIS

The following set of letters is not
random.  They represent a logical and
very familiar sequence.  Bet you can't
fill in the blanks.

O T T F F S _ _ _ _.

The answer:

The blank letters are S E N T.  Still
don't get it? Neither will your friends.
The letters stand for the first letters
of the numbers One, Two, Three etc.

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