I had a friend ask me about this question, and I'm not 100% sure about the data she's using due to the sensitivity/privacy of the data (other than it's medical data). Sorry for not providing context, but hopefully you can think of scenarios or conditions in which the following results would happen.
She was telling me that in her analysis, when she tried to model Y just using variable X, it was not statistically significant. However when she modeled Y using both X and Z, both variables were statistically significant. Why/when does this happen? Is it because of the interaction of variables X and Z, even though X alone would not be statistically significant? Would appreciate any insights!