A transient plane source sensor heats and listens from inside the sample stack, and its information radius grows with time as the probing depth d = 2√(αt). Too early, contact and sensor transients dominate; too late, the thermal wave reaches the sample boundary and the infinite-medium model silently breaks. The legal window lives between those walls — drag the dials and watch it open, shrink, and vanish.
Model: probing depth d = 2√(αt) against a boundary at the sample half-thickness; the early cutoff is an illustrative contact-transient floor ∝ 1/α, the late cutoff is boundary arrival. Real Hot Disk practice sets the window from residual analysis and the standardized procedure; proportions normalized. Schematic teaching tool, not an instrument.