The story about Abraham welcoming his visitors is well-entrenched as part of Jewish wisdom. We should be hospitable, yada, yada. There are probably still cultures in which this sort of absolute and unquestioning hospitality is still the norm - but they're not the culture we live in. If a stranger comes to your door you are more likely to call 911 than to break out the food and drinks, and with good reason.
We understand that reaction, but it's still sad that we have such fears, justified though they may be. Fortunately there are settings in which we can be more open to new relationships, as expressed in this commentary at Ten Minutes of Torah: VaYeira, 5771
Please be sure to click on Vayera in the labels column for more suggestion for thinking about this week's parasha. Also, remember to check Parsha 4 Kids for a ready-to-go trigger you can use with middle school or high school students. And feel free to comment on the idea at either site, yourself or with your students.