While highly ﬂuid pāhoehoe lava can ﬂow like a river, with no more than a slight metallic hiss, rubblelike ‘a‘ā lava moves with a sound like crockery breaking. Much Hawaiian volcanic terrain is streaked with both kinds of congealed lava. It is hard to imagine that shiny, ropy pahoehoe could come from the same source as the dull, jagged clinkers of an ‘a‘ā ﬂow, but lava may emerge from a vent as ‘a‘ā rather than pahoehoe if it has undergone a vigorous stirring during the eruption. The transformation can also occur on the surface as pahoehoe plunges down a steep slope, becoming more viscous from loss of gas. Hawaiians have long summed up the unforgiving nature of ‘a‘ā in sayings such as He ‘a‘ā ko ka hale, which means “Only lava rocks [‘a‘ā] will be found in [that] house,” suggesting an incident of domestic calamity. And yet ‘a‘ā ﬁelds have traditionally been used to grow sweet potato: Hawaiian farmers constructed trails across them made of smooth stepping stones, piled compost in the rubble, and planted seeds.