Logic puzzle: Which octopus is telling the truth? King Octopus has servants with six, seven, or eight legs. The servants with seven legs always lie, but the servants with either six or eight legs always tell the truth. 
One day, four servants met.
The blue one says, “Altogether, we have 28 legs.”
The green one says, “Altogether, we have 27 legs.”
The yellow one says, “Altogether, we have 26 legs.”
The red one says, “Altogether, we have 25 legs.”
What is the colour of the servant who tells the truth?

This is more fundamental than it seems. The way the octopod nervous system works is different from ours. They have no body image, and their legs send no signals back to their brain. So an octopus can only know how many legs other octopuses have. So they can only be sure about what the others say. This means that the only octopus who can have an accurate picture is the one with the most information - the red octopus.
As has been pointed out, the only option with one true answer is 3*7 + 1 * 6. 
The first three octopuses don't know how many legs they, in fact, have, but the fourth octopus knows that the green one has six legs, and sees that the other two have seven, so he knows that there he has seven legs, so he knows he has to lie.
The only octopus that knows the truth is the red one, so the only octopus that actually lies is the red one. The other three do their best, but they cannot know the truth. 
