6. (3 points) Xiaoqiao puts glass balls into a rectangular box, and the number of glass balls she puts in doubles every minute. This way, it is exactly full after 10 minutes. Then, at $\qquad$ minutes, the box is exactly half full.



Please fill in the blank with the correct answer.
【Answer】Solution: According to the analysis, 
$1 \div 2=\frac{1}{2}$ (box), that is, $10-1=9$ (minutes);
Answer: At 9 minutes, the box is exactly half full.
Therefore, the answer is: 9.