Clarification on findstr command with just "search_string" delimiter

Question

So here's the question. I type in findstr "a" in the command prompt then it just shows a blank screen. If I type in a string with an "a" in it, then cmd will return a duplicate of the string I typed in after hitting enter a few times. I just am curious as what is happening.

Example 
B:\Desktop>findstr "a"
a

a

Answer 1

Findstr finds strings in a file. If you don't specify a filename, data is accepted from the keyboard, so you can type to your heart's content. Whenever you enter a string containing the target, findstr will report that string when you press enter. You may terminate the entry-from-keyboard by entering a control-Z keystroke (press control and z simultaneously. This is a standard end-of-file character from the CP/M days, where filesizes were in blocks of 128 bytes, not in bytes, so a control-Z (usually shown as ^Z) was used to indicate a partially-filled block.