This study presents the research that Pramling Samuelsson's keynote at the 32nd annual EECERA conference in Brighton 2024 drew upon. The study addresses an important but understudied feature of early childhood education and care for sustainability (ECECfS), namely engaging children in prospective or what-if thinking. Our reasoning is founded on Vygotskian theorizing on the inherent and dynamic relationship between imagination and real-life experiences, and on distinctions from Play-Responsive Early Childhood Education and Care (PRECEC) between culturally conventional knowing (as is), transformation into play (as if), and changing a premise and thinking about the potential and/or actual consequences of this change (what if). How the latter mode of thinking comes into play and is responded to in read-aloud conversations are analyzed. It is argued that engaging in what-if thinking, implying imagining other possibilities, and thinking about what it would or could entail -- and in extension how to work towards realizing that alternative -- constitutes a critical addition to empirically grounded theorization of ECEC for sustainability.