▲ Work using absorbent cotton or paper.

句中的work 是动词，意为“加工；从事加工”。 using absorbent cotton or paper，意为“使用脱脂棉或纸”。从现在分词和work 的关系看，应该表示“方式”。在using 之前加by 是为了强调该“方式”，除了by 外，还可用through。

问题中的句子，可以变成：Work by using absorbent cotton or paper.

▲ 现在分词可以作方式状语，“介词 + 动名词”也可以用作状语。因此，在不改变句子意义的前提下，增添或减少介词便可以作为区分动名词和现在分词的标志。所谓区别，有介词是动名词，表示的方式更加明显一些；没有介词是现在分词，同样表示方式。请比较：

Using his hybrid rice, farmers are producing harvests twice as large as before. 通过使用他的杂交稻种，农民现在生产的粮食是以前的两倍。（using 是现在分词）

By Using his hybrid rice, farmers are producing harvests twice as large as before. （using 是动名词）

They also express their feelings using unspoken language through keeping physical distance, actions or posture. 他们还通过使用无声语言，借助保持距离、动作和姿势，来表达感情。（using 是现在分词）

They also express their feelings by using unspoken language through keeping physical distance, actions or posture.（using 是动名词）

He must be able to use the foreign language, forgetting all about his own. 他一定要做到使用外语时，忘掉本族语。（forgetting 是现在分词）

He must be able to use the foreign language through forgetting all about his own. （forgetting 是动名词）

▲ 以下各句中的by 都可用可不用，意思保持不变：

(By) doing extra homework, he soon got ahead of his class-mates.  他靠多做家庭作业，很快在班上名列前茅。

I worked off my debt (by) doing odd jobs. 我用打零工的方式来偿清债务。

You wronged him (by) saying that he had lied. 你说他说谎是冤枉了他。

She excused her absence (by) saying that she was ill. 她为她的缺席申辩,说是病了。

He ended off his speech (by) telling a very funny joke. 他讲了一个非常滑稽的笑话结束了讲话。