Now run:

$ MYVAR=hello
$ ./myvar2.sh
MYVAR is:
MYVAR is: hi there

$ MYVAR=hello
$ ./myvar2.sh
MYVAR is:
MYVAR is: hi there

It's still not been set! What's going on?! 
When you call myvar2.sh from your interactive shell, a new shell is spawned to run the script. This is partly because of the #!/bin/sh line at the start of the script, which we discussed earlier. 
We need to export the variable for it to be inherited by another program - including a shell script. Type:

$ export MYVAR
$ ./myvar2.sh
MYVAR is: hello
MYVAR is: hi there

$ export MYVAR
$ ./myvar2.sh
MYVAR is: hello
MYVAR is: hi there