填 that ,這是固定語法需求
She said that ...
I found that ...
We agreed that ...
都是固定的結構
They found that things are often famous for only a short time
that 是連詞,引導賓語從句 that thing are often famous for only a short time
其他類似例句：
She said that I was late
I found that some food can actually harm you
We agreed that we would meet again tomorrow
我把原句改通順一點,另外,這樣說更地道：
They have found that fame often lasts only a very short time.
他們發(fā)現(xiàn),聲譽很常是曇花一現(xiàn)