Is that cake for eating or just for looking at?

网友这句话的意思是：那蛋糕是用来吃的，还是用来看的？

动名词作定语修饰“东西、物品”，一般表示“目的或用途”。有两种表达方式：

1. 直接把动名词放在名词之前作定语：a swimming pool

2. 用 for doing 放在名词之后作定语：a pool for swimming 

无论哪种方式，动名词都不用被动形式。因为这里的用途、作用，实际表明人的行为，而不考虑该事物与动词是否是主动、被动关系。

a walking stick → a stick for walking

a sleeping car → a car for sleeping

a waiting room → a room for waiting

a swimming pool → a pool for swimming

a washing machine → a machine for washing clothes

a writing desk → a desk for writing

a reading room→ a room for reading

a bathing cap→ a cap for bathing

a changing room→ a room for changing clothes

a sitting room→ a room for sitting

a watering can→ a can for watering flowers

a guessing game→ a game for guessing words

an ironing board→ a board for ironing clothes

a collecting tin→ a tin for collecting money

a hiding place→ a place for hiding

a racing bicycle→ a bicucle for racing

an operating table→ a table for operating

building materials → materials for building

a diving board→ a board for diving

reading materials → materials for reading

a closing speech→ a speech for closing a meeting

working hours→ hours (time) for working

a smoking room→ a room for smoking

a dancing hall → a hall for dancing

a washing machine → a machine for washing clothes