Trak Corporation incurred the following costs while manufacturing its bicycles.
| Bicycle components 
 | $100,000 | Advertising expense | $45,000 | 
| Depreciation on plant | 60,000 | Property taxes on plant | 14,000 | 
| Property taxes on store | 7,500 | Delivery expense | 21,000 | 
| Labor costs of assembly-line workers | 110,000 | Sales commissions | 35,000 | 
| Factory supplies used | 13,000 | Salaries paid to sales clerks | 50,000 | 
Explain the basic difference in accounting for product costs and period costs.