The Value of Expertise

An apartment building maintenance man was rousted out of bed one cold night because one of the tenants needed more heat.
The man went to the apartment, sized up the situation, took out a rubber-headed mallet, and with one WHACK started the heat flowing again through the radiator.
"How much do I owe you?" the tenant asked. "$25," was the reply. "What? $25 for one whack of your mallet?" "No," said the maintenance man. "The whack only cost fifty cents. Knowing WHERE to whack cost $24.50."