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.
Hyon Chol-hae was born in 1934.
Chan-Hae Lee was born in 1945.
Ryu Ji-Hae was born in 1976.
Song Hae-seong was born in 1964.
Min Hae-Kyung was born in 1962.
Yi Hae-won was born in 1919.
Lee San-hae died in 1609.
Lee San-hae was born in 1539.
Choe Ryong-hae was born in 1950.
Park Hae-il was born in 1977.