Numbers 1 to 20
One (1), two (2), three (3), four (4), five (5), six (6), seven (7), eight (8), nine (9), ten (10).
Eleven (11), twelve (12), thirteen (13), fourteen (14), fifteen (15), sixteen (16), seventeen (17), eighteen (18), nineteen (19), twenty (20).
Notice the pattern from 13: three+teen = thirteen, four+teen = fourteen, and so on. Eleven and twelve are special.