First, let's consider the last question directly. The change of the tray is zero. Let's draw three pictures first, as shown in Figure 3. Because the volume of boiling water discharged by wood blocks is different, the water level is also different. First of all, before cutting the thin thread, the wood block is subjected to gravity G and buoyancy F, but the gravity of F floating is large, so it is transferred to the F pulling force at the bottom of the cup. Then we went back to the beginning. After cutting the rope, as shown in Figure 2, it is the pink part. As shown in Figure 3, the buoyancy of the wood block is pink and blue, and the offset gravity is pink, so the F-pull is the gravity of the water blue part, but after cutting the thin line, the blue part disappears and the F-pull disappears. Moreover, in Figure 2, the gravity of the wood block and the buoyancy of the water are equal and offset each other, but the pressure of the boiling water of the wood block on the bottom of the cup, that is, the pink part, is not offset. It is not easy for a baby. Ho ho.