I'm a non-native teacher of English and I work with adults.
"The sink is next to the toilet"
"The sink is behind the toilet"
I feel we should use "next to" because the toilet doesn't cover the front side of the sink and some of my students agreed with me. That very student, however, is an engineer and he sees geometry everywhere. He said that if a person was standing on the rug they would see the toilet covering the most part of the sink, so "The sink is behind the toilet".
Now I'm pretty confused. Which preposition would you use and why?