I said: well...I've never thought about that....
then I found this explanation to this queston.
Understanding Chinese Pinyin: érhuà (儿化)
In Chinese Pinyin System, the suffixion of a non-syllabic 儿(r) to nouns and sometimes verbs or adjectives, causing a retroflexion of the preceding vowel, typical of the pronunciation of standard Chinese and of some dialects.
In Chinese érhuà words, the two Chinese Hanzi (Chinese characters) should pronounce as one syllable. For example:
花儿(flower) should pronounce as one syllable "huār";
活儿(job, work) should pronounce as one syllable "huór".
érhuà used for the following occasions:
1. Added to nouns to express smallness. 女孩儿(girl)→nǚháir
2. Added to verbs to form nouns. 画儿(drawing)→huàr
3. Added to adjectives to form nouns. 热闹儿(fun)→rè'nàor
4. Added to concrete nouns to change them into abstract nouns.
5. Added to nouns to form nouns with defferent meaning.
白面(wheat flour) : 白面儿(heroin)→bámiànr
6. A verb suffix very much restricted in use.
玩儿牌(play cards)→wánr pái.
Well..I can't remember all that myself...let alone my foreign students...The thing is...you don't have to know too much~
Just SPEAK!! right?