Despite its ubiquity in residential middens at many North American archaeological sites, thermally modified rock (TMR) is among the least studied elements of the archaeological record. TMR assemblages, however, may provide key insights into routine cooking practices, patterns of refuse disposal, and midden formation processes. This article outlines the results of experimental research aimed at understanding the conditions by which TMR assemblages were created in residential settlements in the Pacific Northwest. We present baseline data addressing the thermal properties of the hearth, the rate and circumstances of cobble fracturing, the extent to which different kinds of cobbles break when exposed to heat for varying durations, and the effectiveness of hot cobbles at achieving cooking temperatures.