Indicating end of Standard Input

How does one indicate that one has finished entering test in stdin?

For example, let's say that I wish to encrypt 'blue' using MD5 (I know MD5 is unsecure, but just for this example). I tried

user$ blue | md5

which I was led to understand is how one pipes input to stdin, but it doesn't work right.

But if I just enter

user$ md5

I can enter the word 'blue'. But how do I indicate to md5 that I'm finished entering text?

are you talking about getting an md5sum for a piece of text?

if so run the md5sum command

type your text, when finished move to a new line by pressing return

press CTRL-D to end your input.

 user$ md5sum
 blue
 CTRL-D