There is a safety mechanism in place [to ensure the perambulator doesn't turn back into a purse with a baby in it] : if anything weighing more than a pound and a half-about the weight of a three-volume novel-is in the carriage of the perambulator, it will not transform.
Lev A.C. Rosen, All Men of Genius
humorous, importance-of-being-earnest, perambulator, silly