Answer: Where there is smoke there's fire.
Step-by-step explanation:
An adage is referred to as a wise saying. It's a traditional saying that expresses a truth about a particular thing.
The adage in the story is "Where there is smoke there's fire". This means that there's a rumour about something and therefore there must be some truth about that thing.
Here, the narrator used the adage in explaining that the yogurt may be spoilt and that the sister needs to be careful.