Question

Shyla conducted an experiment. she went to the student union and asked people if they would like to be subjects in her study. she decided to ask only those people who smiled at her. is this an appropriate way to select subjects?

Answer 1

No!
It's biased. Grumpy people have a right to have their opinions known just as well as cheerful people. Grumpy looks are momentary. They can be brought about (especially at a University) by math problems that are illusive, by girl friends (or boy friends), that are illusive, by something for lunch that was barely edible, by any number of things. 

She should ask people on some sort of other scale that is more random.