How a man can be his own grandfather

"There was a widow [Anne] and her daughter [Jane], and a man [George] and his son [Henry]. The widow married the son, and the daughter married the father..."

I believe I remember Mark Twain once writing (or asserting in a speech) that a man could be his own grandfather. The case illustrated is said to have occurred in 1846, and may have been the one Twain was alluding to.

It is, of course, a matter of semantics rather than genetics. The explanation is short and is available at the Futility Closet.

Update: Hat tip to Ryan for noting that this topic is discussed in Wikipedia.