That said, I think it's worth discussing the rules of time travel or "the time hole" as my colleague Ross and I put it. So if you've followed the Terminator movies, and, more recently, Terminator: The Sarah Connor Chronicles, the rules of the time hole are fairly easy to infer.
Things can go through the time hole if:
- They are made of meat
- They are wrappped in meat
- They look very much like they are made of meat
- If several things meeting the above rules go through all at once, something else may sneak through behind them.
In Terminator 2, we see the T-1000, which is constructed of a "mimetic poly-alloy," i.e., liquid metal, travel through time. Clearly, the T-1000 qualifies under rule #3 as it looks, very convincingly, like it is made of meat. Similarly, in Terminator 3, we see the T-X come back in time. The T-X is much like the T-1000, except hot.
Later, in the The Sarah Connor Chronicles, we see Cameron (a terminator wrapped in meat), John, and Sarah (both made of meat) travel back in time. However, just before the time hole closes, Cromartie's severed head (devoid of its meat wrapping) flies through after them.
I checked all terminator-related facts presented above on wikipedia.org.