The Post Correspondence Problem (PCP)

[@brian_kirk]

This was excellent, thank you for taking the time to make it and post it.

[@VoxFruit]

this is actually pretty good. dominos makes more sense too

[@robert8976]

the first example might be wrong. The number of letter different between top and down. How can rows be same?

[@JanekBogucki]

I love the graphics. The artificial voice is distracting however.

[@JoshKings-tr2vc]

Looks like series to me.