Sheets Object Syntax - Selects Cell In Each Sheet Within The Book

I'm trying to write a macro that selects cell A1 in each sheet within the book. I've written the following code:

Code:
Sub a1()
For i = 1 To Sheets.Count
Sheets(i).Cells(1, 1).Select
Next i
Sheets(1).Select
End Sub

However, it produce this message: "Run-time error '1004': Select method of Range class failed."

If I slightly modify the code to

Code:
Sub a1()
For i = 1 To Sheets.Count
Sheets(i).Select
Cells(1, 1).Select
Next i
Sheets(1).Select
End Sub

It works fine. I'm curious why this happens.