哪儿
nǎ r
where?
wherever; anywhere; somewhere; (used in rhetorical questions) how can ...?; how could ...?
which (哪) + place suffix (儿) = where?
哪 uses 口 (mouth, asking) while 那 gives the sound. The suffix 儿 softens and localizes the question, turning 'which' into 'where' in spoken Mandarin.
你的书包在哪儿？
nǐ de shū bāo zài nǎ r？
Where is your school bag?
他哪儿都去过，经验很丰富。
tā nǎ r dōu qù guò，jīng yàn hěn fēng fù。
He has been everywhere, so he has a lot of experience.