The temperature of an object will decrease if it radiates more energy than it absorbs. If an object is a good conductor, heat will be conducted from the earth into the object to replace the heat that is radiated away. Put the beverage in the freezer. Newton s law of cooling says it will cool faster if there is a larger temperature difference. The silver surfaces reflect radiated heat so it does not go through the thermos.
Water is a better thermal conductor than air.
Thus the water conducts heat away from your body better than air does and you feel colder in the water. If the block of wood and metal are both at the same temperature as your body, no heat will be conducted to or from your body. Then it won t matter which material is a better conductor because there is no temperature difference to cause heat to be conducted. Your body can supply heat to your tongue as fast as it losses heat, so your tongue doesn t cool and freeze to the wood. Yes, wood still has poor heat conductivity when it is hot.