Mr. P and Mr. Q have sufficient reasoning ability. On this day, they are being interviewed by reasoning. They know that there is something in the drawer of the desk.

Analysis: 16 card is as follows

Hearts: a, q, 4

Spades: J, 8, 7, 4, 3, 2

Grass flowers: k, q, 6, 5, 4

Diamonds: a, 5

(1) Note: Mr. P only knows points. According to Mr. P, we can know that this card must have the same number of other N cards (n = 1, 2, ...), so we can know that this card may be: A, Q, 5, 4.

(2) Note: Mr. Q only knows about suits. According to Mr. Q's words, even if Mr. P doesn't say anything, Mr. Q knows that Mr. P can't guess, so it can be inferred that any card in the suit to which this card belongs has been repeated in other suits. So exclude colors: spades and flowers.

(3) At this time, Mr. P heard Mr. Q say "I know this card now", which means that the number of points in this card is unique among hearts and diamonds, so the possibility of A is ruled out.

(4) Note: Mr. Q's second sentence is the key point. After listening to Mr. P, Mr. Q said, "I also know this card." This shows that in the suit of this card, only two cards are duplicated in other suits, and one of them must be A, otherwise it cannot be judged. In addition to A, there is a repetition of A and 4 in the heart, so it can be inferred that this card must not be a heart.

(5) In the box, according to the conclusion in (3), the answer can obviously be obtained: box 5.