A thermometer shows 15∘C at 8:00 a.m. and 23∘C at noon. The temperature T(t), measured in degrees Celsius, is known to be differentiable for 8≤t≤12, where t is time in hours after midnight. Explain in one or two sentences why there must be a time between 8:00 a.m. and noon when T(t)=20∘C.