Safe as houses

"" was an English language expression that befuddled the Doctor — apparently until his eighth incarnation. While in the interzone of the Divergent Universe searching for Caerdroia, he was once split into three bodies. One of his three selves finally figured out that the expression originated in a 19th century recession in which the only financial security was in real estate. (AUDIO: Caerdroia)

The expression also made no sense to nine-year-old Clara Oswald, who noted that houses could be "knocked down, flooded, felled by earthquakes, set on fire, taken to Oz by tornadoes..." (PROSE: Normality)