They can smell a change in the chemicals their body makes.
No words can "actually" answer this question. Your going to have to find out yourself or ASK MILLY!!!
Sim Hae-In was born in 1987.
Lee San-hae died in 1609.
Lee San-hae was born in 1539.
Choe Ryong-hae was born in 1950.
Lee Kyung Hae died in 2003.
Lee Kyung Hae was born in 1947.
Min Hae-Kyung was born in 1962.
Yi Hae-won was born in 1919.
Park Hae-il was born in 1977.
Hyon Chol-hae was born in 1934.
Chung Hae-Won was born in 1958.