A researcher tests whether a sports drink improves dunking ability in 8th graders, but students vary wildly in height, from 5 ft 8 in to 7 ft 2 in. That height difference could explain any results instead of the drink. You need to control confounding variables to know what actually caused the effect.
One of the things to think of in internal validity, and this typically is involved in a quantitative study. Have you controlled for external or extraneous variables or inputs? For example, you have given 8th graders a sports drink and you want to see how they can dunk a basketball and that's it. You say, okay, I've given these 8th graders this sports drink and now can they dunk a basketball? Well first, what's the height of these 8th graders? Are all of them 5 foot 8? Are all of them 7 foot 2? Or is the variation in there? Because if I'm 7 foot 2 I start a lot closer to be able to dunk than the 5 foot 8 person. So it may not be the sport drink. So we want to think about any variables that can impact the outcome which then impacts your results. Drop the word help in the comments and I'll send you a link to my free dissertation help community.
