about the usage of prepositions "in" "on" "at" Can I differentiate the two by the principle that when subject A just touches the surface of subject B, and does not cause B to sink or bend, we say A is on B; e.g. a picture on the wall, an apple on the table... whereas when A causes B to sink, or some part of A inserts into B, then we say A is in B?; e.g. a baby is sleeping in bed; a tree grows in mountain top... 2, about being * a place, Is that we use "in" a very big place (e.g. in China, in East Asia, in England, in London), "at" a smaller place (e.g. at a kindergarten, at a plaza, at coutryside)? but it seems OK to say "I am in classroom now" "I am in 201 office"... confused >< Many thanks!