system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are dogs' ears all floppy when they're puppies but stiffen up when they're adults?
gpt
The reason why ears are floppy in pups and erect as adults (in those prick eared breeds) is because the ear cartilage is soft and not fully formed. The cartilage doesn't start to harden or set until 16 weeks or so which is why their ears can be up one day or opposite ears over a few days etc.  
Now as for those that are cropped, the minor surgery is performed no younger than 8 weeks and no older than 16 weeks as it needs to be soft in order to get the ideal shape and they be fully erect by the time they finish teething.