dropdown 2 value change based on the dropdown 1 value selected?

HI all,

I have 2 dropdown fields in my module pool program. Based on the value selected from 1st dropdown, the values for the second dropdown should be changed dynamically. So, values for second dropdown is dynamically changed based on the value selected in first dropdown. I dont want to perform any other action (any button click, etc) for this change to happen.

Can someone tell me in which module i need to write the logic, so that the values for second dropdown will be automatically changed based on the option selected in first dropdown.