Simulate the Wireworld cellular automaton
APL, Dyalog (131)

{h t c←1↓⍵∘=¨F←' htc'⋄⎕SM∘←1 1,⍨⊂N←F[1+⊃+/H h(t∨c≠H←c∧S↑1 1↓1 2∊⍨⊃+/+/K∘.⊖(K←2-⍳3)∘.⌽⊂¯1⌽¯1⊖h↑⍨2+S←25 79)×¨⍳3]⋄∇N⊣⎕DL÷4}↑{79↑⍞}¨⍳25


The output is displayed in the ⎕SM window. The simulation runs endlessly. The grid is 79x25 because that's the default size of the ⎕SM window. The electron head is h, tail is t, copper is c. The program reads the starting configuration from the keyboard as 25 lines. 

Explanation:


↑{79↑⍞}¨⍳25: read the 79x25 grid
h t c←1↓⍵∘=¨F←' htc': get three matrices, one with the heads, one with the tails and one with the copper. Also set F to the string ' htc'.
⎕SM∘←1 1,⍨⊂N←F[1+⊃+/...×¨⍳3]: The "..." part is a vector of length three, where the elements are matrices showing the new heads, tails, and copper respectively. The heads are multiplied by 1, the tails by 2, and the copper by 3, then we sum these matrices together and add one, giving a matrix of indices into F. N becomes the new state, in the same format as the input, and it is shown on the ⎕SM screen starting from the top-left corner. 
¯1⌽¯1⊖h↑⍨2+S←25 79: Add a border of blanks to h by growing it by two rows and columns, then rotating it one right and one down.
⊃+/+/K∘.⊖(K←2-⍳3)∘.⌽⊂: Rotate the matrix in all eight directions and then sum the resulting matrices together, giving the amount of neighbours that are heads on each position.
1 2∊⍨: Set only those positions to 1 which have 1 or 2 neighbours.
S↑1 1↓: Remove the border we added earlier.
H←c∧: The new heads are all those copper cells that have 1 or 2 head neighbours.
t∨c≠H: The new copper cells are all old tails, and all old copper cells that have not become heads.
H h(...): The new heads are H as calculated above, the new tails are the old heads, and the new copper cells are as calculated above.
∇N⊣⎕DL÷4: Wait 1/4th of a second, then run the function again on N.