Describe the situation (in terms of molecular speed of the water) in which you have a pot of boiling water, you take off the lid for one second and then place it back on the pot
Initially, there was steam contained inside the closed pot. Once the lid was removed, the steam escaped. This causes a reduction in the number of molecules in the system as well as a loss in kinetic energy. Placing the lid back on, the water will re-equilibriate. This will cause the average molecular speed of the system to decrease. Also consider that when the energy is lost from escaping steam, the temperature goes down for the system as well, since energy is proportional to temperature.