Before I attempt to explain concurrent computation in ToonTalk and why it is difficult to extend sequential languages to be concurrent, I'll explain the relationship between concurrency, parallelism, and distributed computing.
Parallel processes are processes that are running within the same computer system.
The system typically has more than one processor. Distributed processes are processes that typically run on different computer systems connected via a network. The distinction between parallel and distributed is important because distributed computing is typically across trust or organizational boundaries. Hence only distributed computing need be concerned with security, resource allocation, hardware failure recovery, and the like. In discussions where the distinction is not important, I refer to concurrent processes, i.e. processes that might be either parallel or distributed.