Students that I've worked with have been discovering that not all LEDs play well with one another, but I haven't figured out a simple way to explain forward current, other than to say that "electrons are lazy." (Although this amazing experiment, which I just discovered on on Jie Qi's flicker feed, is an incredibly useful visual representation!)
Although my understanding of resistance, current, and voltage is not as robust as I'd like it to be, I'm starting to understand how it all comes together. My goal is to understand it well enough that I can figure out a way to explain it as simply, accurately, and confidently as possible.
Here are a couple of experiments I recently conducted to demonstrate a phenomenon that students experience when they are trying to use a blue (3.2 V) and red (2.2 V) LED within the same circuit. The LED with the lower forward voltage (red) will receive the current without a resistor. I've demonstrated how a couple of different types of resistors impact this phenomenon below.
Of note, Circuit Stickers (not shown here) have resistors built in, so your students wouldn't encounter this issue.