Two Russian mathematicians meet on a plane.
'If I remember correctly, you have three sons', says Ivan. 'What are their ages today?'
'The product of their ages is thirty-six, says Igor, 'and the sum of their ages is exactly today's date.'
'I'm sorry, Igor, Ivan says after a minute, 'but that doesn't tell me the ages of your boys.'
'Oh, I forgot to tell you, my youngest son has red hair.'
'Ah, now it's clear', Ivan says. 'I now know exactly how old your three sons are.'
How did Ivan figure out the ages?