Giants Kolodej and Beaver speak only the truth on some days and only lie on others. Kolodej speaks the truth on Monday, Friday, and Sunday, and lies on the other days. Beaver speaks the truth on Wednesday, Thursday, and Friday, and lies on the other days.

1. Determine when Kolodej can say: "I spoke the truth yesterday."
2. One day, both said: "I lied yesterday." On which day was this?

(M. Volfová)

Hint. Help yourself with a clear table, from which it would be apparent when each giant lies and when they do not.
Information from the assignment for clarity is entered into the table:

|  | Kolodej | Bobr |
| :---: | :---: | :---: |
| Monday | + | - |
| Tuesday | - | - |
| Wednesday | - | + |
| Thursday | - | + |
| Friday | + | + |
| Saturday | - | - |
| Sunday | + | - |

1. Kolodej can certainly say "yesterday I told the truth" on Monday, because he speaks the truth on Mondays and did so on Sundays (yesterday). There is no other pair of consecutive days when he speaks the truth. If Kolodej were to say the same statement on a day when he is lying, it would mean that the previous day he was actually lying. This could only happen on Wednesday or Thursday. Therefore, Kolodej can claim "yesterday I told the truth" on Monday, Wednesday, or Thursday.
2. Similarly, if one of the giants says "yesterday I lied," it can be either on a day when they are telling the truth and the previous day they lied, or vice versa. Kolodej can make this statement on Tuesday, Friday, Saturday, or Sunday; Bobr on Wednesday or Saturday. The only day when both can claim "yesterday I lied" is Saturday.