Key is the fifth episode of Alphablocks Season 1, and the 5th overall.

C and K is the main character, and A, C, E, G, I, K, L, O, R, S, Y, CK, EY, Magic E, and A_E appear in the episode.

At first, C is seen cracking cracks to her sound, then she meets her brother K, who is kicking the ball to his sound while R counts the keep-ups. C thought K is copying her. C's cracks ruin K's keep-ups. C and K decide a contest. K does it on E and Y. A large cage falls onto everyone except E, G, and Y. As K takes the key to the locked door, there is no keyhole. So, C and K join together to make the CK sound.

This episode teaches the letters C and K.