6. (6 points) The figure is a rectangle. To divide it into 7 parts, at least $\qquad$ straight lines need to be drawn.


【Analysis】Two lines divide a square into 4 parts, the third line intersects with the first two lines, adding 3 more parts, making a total of 7 parts; the fourth line intersects with the first 3 lines, adding another 4 parts, making a total of 11 parts; the fifth line intersects with the first 4 lines, adding another 5 parts, as shown in the figure below.

【Solution】Solution: $1+1+2+3=7$
Answer: By drawing 3 lines on a rectangle, it can be divided into a maximum of 7 parts.
Therefore, the answer is: 3.