Making coffee on Shabbos using 2 cups assuming no essence was made: Pour water from kettle into cup #1, pour water from that cup into cup #2 that has coffee placed into it. The question is: CAN ONE USE STEROFORM CUPS? These cups retain heat but do not dissipate heat.
Answer:
Yes, styrofoam cups may be used. While these cups retain more heat than the average cups, the water does cool down from kettle to cup 1 and to cup 2, and as such has the status of kli shlishi, the third vessel from the original. When the Talmud sets the criteria, the only condition is that it be the next vessel, and does not stipulate the material the cup is made from. As such we can assume that even a minimal reduction is sufficient.
