A flask of coloured water was plunged into a beaker of hot water. Although the water lvl was expected to rise, it was seen falling first before rising. Why?

Determine the fraction of the magnitude of kinetic energy lost by a neutron (m1 = 1.01 u) when it collides head-on and elastically with a target particle at rest which is 21h (heavy hydrogen, m = 2.01 u).