fairness
Fair cake cutting
You cut the cake. The other person chooses first.
Two people want to share a cake. One person cuts the cake into two pieces. The other person chooses which piece to take first. The cutter knows the chooser will take the piece they prefer, so the cutter is motivated to make the pieces equal.
You cut the cake. The other person will choose their piece first. How do you cut it?
Because the other person chooses first, any uneven cut leaves you with the smaller piece.
The only way to guarantee yourself half is to cut the cake into two equal pieces.
This "I cut, you choose" rule produces a fair division without needing a referee. The person who cuts has a strong reason to be precise.