Priority of Controls placed over each other

If I first place a Textbox on a form and then place a Datagrid on the form the Datagrid has priority i.e. if I move the Datagrid over the Textbox the Datagrid hides the Textbox. However if I place the Datagrid on the form first and then place the Textbox on the form and again move the Datagrid over the Textbox - the Textbox shows through the Datagrid i.e. the Datagrid appears transparent. Is there a property that determines this behaviour? I want the Datagrid to hide the underlying Textbox. I can solve this in one of 2 ways: 1) I can place the Datagrid on the form last or 2) I can programatically make the underlying Textbox invisible. However I am curious to know what determines this behaviour.