Put your sentence in a spoiler, and say what kind of word the blank should be.
For example,
Person 1 wrote:Plural noun.
The person that posts after you chooses a word, opens the spoiler, and posts it in their um...post. Then, they post the kind of word and the sentence of their own.
The Next Person wrote:How many tubes can you fit on the ropf.
Verb (gerund).
I'll start.
Noun.
The next person should post a noun, the kind of word they want, and the rest of their sentence.
The only rule: Don't open the spoiler until you've chosen a word. The only exception to this rule is that if you open your spoiler and see that the sentence it makes is inappropriate or offensive, you may change your word.
If you open the spoiler before you choose a word, I will be standing over your shoulder and graffiti your face. Oooh burn!