. Pierre says: «The day before yesterday I was 10 years old. Next year, I will celebrate my 13th birthday.» What day is it?
. Next year, Pierre will celebrate his 13th birthday. This year, he will therefore have or has already celebrated his 12th birthday. So he is either 11 or 12 years old. Since he was 10 the day before yesterday, he is actually 11 now. This implies two things: (1) he had his birthday yesterday or today; (2) he hasn't celebrated his birthday yet this year. Thus, his birthday was yesterday and it was last year. So yesterday was December 31 and today is January 1.