6. 15 cards, each card has 3 different Chinese characters, any 2 cards do not have exactly the same characters; among any 6 cards, there must be 2 cards that have a common character. Question: What is the maximum number of different Chinese characters on these 15 cards?
【Analysis】Considering cards 1 to 6, at least 2 Chinese characters are repeated, let's assume the first card has a repeated character. Considering cards 2 to 7, at least 2 Chinese characters are repeated, let's assume the second card has a repeated character, ..., considering cards 10 to 15, at least 2 Chinese characters are repeated. In this way, there are at least 10 repetitions. The 15 cards have a total of 45 Chinese characters, so there are at most 35 different Chinese characters.

【Solution】According to the problem analysis: 1 to 6 cards, at least 2 Chinese characters are repeated, let's assume the first card has a repeated character,
2 to 7 cards, at least 2 Chinese characters are repeated, let's assume the second card has a repeated character,
10 to 15 cards, at least 2 Chinese characters are repeated,
In this way, there are at least 10 repetitions,
Also, the 15 cards have a total of 45 Chinese characters,
$$
45-10=35 \text { (characters), }
$$

Answer: There are at most 35 different Chinese characters.