1.2' s birthday is the same day because there are 365 or 366 days in a year, and there are 367 classmates, two of whom must be in the same temple and at least two boys in the same month. There are 12 months in a year, 18 students. At least one or two girls were born in the same month.

Pigeon hole principle: put m objects in (M- 1) drawers, and at least one drawer has 2 objects.

2。 4 pieces

3。 three