# Can you solve the alien probe riddle? – Dan Finkel

The discovery of an alien monolith

on planet RH-1729 has scientists across the world

racing to unlock its mysteries. Your engineering team has developed

an elegant probe to study it. The probe is a collection

of 27 cube modules capable of running all the scientific

tests necessary to analyze the monolith. The modules can self-assemble

into a large 3x3x3 cube, with each individual module

placed anywhere in the cube, and at any orientation. It can also break itself apart

and reassemble into any other orientation. Now comes your job. The probe will need a special

protective coating for each of the extreme

environments it passes through. The red coating will seal it against

the cold of deep space, the purple coating will protect it

from the intense heat as it enters the atmosphere of RH-1729, and the green coating will shield it

from the alien planet’s electric storms. You can apply the coatings to each of

the faces of all 27 of the cubic modules in any way you like, but each face can only

take a single color coating. You need to figure out how you

can apply the colors so the cubes can re-assemble themselves

to show only red, then purple, then green. How can you apply the colored coatings

to the 27 cubes so the probe will be able

to make the trip? Pause here if you want

to figure it out yourself. You can start by painting the outside

of the complete cube red, since you’ll need that regardless. Then you can break it into 27 pieces,

and look at what you have. There are 8 corner cubes,

which each have three red faces, 12 edge cubes,

which have two red faces, 6 face cubes, which have 1 red face, and a single center cube,

which has no red faces. You’ve painted a total of 54 faces red

at this point, so you’ll need the same number of faces

for the green and purple cubes, too. When you’re done,

you’ll have painted 54 faces red, 54 faces green, and 54 faces purple. That’s 162 faces, which is precisely

how many the cubes have in total. So there’s no margin for waste. If there’s any way to do this,

it’ll probably be highly symmetrical. Maybe you can use that to help you. You look at the center cube. You’d better paint it half green

and half purple, so you can use it as a corner

for each of those cubes, and not waste a single face. There’ll need to be center cubes

with no green and no purple too. So you take 2 corner cubes

from the red cube and paint the 3 blank faces of 1 purple, and the 3 blank faces of the other green. Now you’ve got the 6 face cubes

that each have 1 face painted red. That leaves 5 empty faces on each. You can split them in half. In the first group,

you paint 3 faces green and 2 faces purple; In the second group,

paint 3 faces purple and 2 green. Counting on symmetry, you replicate these piles again

with the colors rearranged. That gives you 6 with 1 green face, 6 with 1 red face, and 6 with 1 purple face. Counting up

what you’ve completely painted, you see 8 corner cubes in each color, 6 edge cubes in each color, 6 face cubes in each color, and 1 center cube. That means you just need 6 more edge cubes

in green and purple. And there are exactly 6 cubes left,

each with 4 empty faces. You paint 2 faces of each green

and 2 faces of each purple. And now you have a cube that’s perfectly

painted to make an incredible trip. It rearranges itself

to be red in deep space, purple as it enters RH-1729’s atmosphere, and green when it flies through

the electric storms. As it reaches the monolith, you realize you’ve achieved

something humans have dreamt of for eons: alien contact.