一老外问我关于这个问题,我怎么回答?

你就跟他说“：这 2个发音不一样，就象音标中的重读标志，中国人把语调

分为4种，不过..你（指老外）不用知道，因为你在中国说话中国人能听懂~因

为他们都很聪明~”

对外汉语有专门的解释，你可以简略地说,the language of chinese is a musical one. each spelling has the same way of pronouciation while five different tones.
如果有机会，也可以向他介绍一点汉语学习的书本。现在比较主流的是商务印书馆出的一套吧

This is just two words which have same pronounciation but differen't spelling and meaning. Both of them can be used either singly or with other word to form a new meaning. This is the same as En, like "for" and "four", their pronounciation is same, while meaning or function is different:):).
我觉得这样回答就可以了：）

In Chiese, there are five tones. Same syllables with different tones may have totally different meanings. The "zi"in "Zi Mu Biao" is the falling tone, while "zi" in "Zi Se" is the falling-rising tone.

in chinese, there are five tones, each one indicates different pronouciation( bad spelling); moreover, different characters may have the same pronouciation(again,,,), but represent different meanings. ...using dictionary may help..