Because: a teacup with a bottom can only be inverted odd times, so if all seven teacups are inverted, it needs to be inverted odd times seven times, and the total is the sum of odd numbers, which is odd. However, every time we rotate four cups, no matter how many times we operate them, the total number of times the seven cups rotate is a multiple of four, which is even and contradictory. Therefore, no matter how many times you operate it, it is impossible to make all the cups face down.