1. We can imagine two potatoes as two air masses that can overlap and enter each other's interior. When two air mass potatoes are approaching, there must be a moment when the two air mass potatoes cross. At this time, the intersection lines of their surfaces are completely coincident, and the intersection lines are on the surfaces of two potatoes, so there are two congruent curves on the surfaces of the two potatoes.

Note: This method is not a mathematical proof, but an intuitive demonstration.

2. Set 30 kinds of feed as 1, 2, 3, …, 30, and 5 mice as A, B, C, D and E respectively.

1 feed A, 2 feed B, 3 feed C, 4 feed D, 5 feed E, 6 feed AB, 7 feed AC, 8 feed AD, 9 feed AE, 10 feed BC, 1 1 feed BD, 12 feed BE and/. 18 feed ABE, 19 feed ACD, 20 feed ACE, 2 1 feed ADE, 22 feed BCD, 23 feed BCE, 24 feed BDE, 25 feed CDE, 26 feed ABCD, 27 feed ABCE, 28 feed ABDE, 29 feed ACDE, 30.

According to the above scheme, each feed corresponds to a different group of mice, and you will know which feed is toxic by seeing which group of mice will die.

Note: In fact, five rabbits can judge 32 kinds of feed at most. No.31feed is fed to all five mice, and No.32 feed is fed to all five mice, so each feed corresponds to a different group of mice, and you can know which feed is toxic by seeing which group of mice will die.