Instruction: Extract information from the supporting document to answer the given question.

Input:
Question: Why does mold keep growing on my butter?  I have followed the directions carefully and I have mold. REALLY NOT HAPPY
Document: If you keep real butter on your table or counter un-refrigerated this is the trick! My wife had one just like it when she was growing up on the farm!
This is a great item. It looks good in the kitchen and does exactly what it's designed to do...keep butter soft and readily available. I discovered though that you need to keep it away from the heat source otherwise the butter will get mold on it. The further away the better. i bought one for my Mom too, and she loves it.Great item at a great price....
Been using it for a month and loved it. Forgot to change water for a week and know I had air pockets in the butter... Opened and found mold. Going to wash and try again.
fits well together. I did get mold in the water when I didn't change it often enough. Probably once a week it prefered. I have had trouble with butter absorbing other flavors and this worlk.
It came nicely packaged and in a timely manner. I have used it now for a couple of weeks. Love the idea of a butter crock, but this one doesn't seal well enough. I have found ants in it and today I found mold. So I am a little disappointed in the product.
Not very practical. Although followed the instructions to the dot, butter kept falling down in the water. Even started with chilled butter but when it came to room temperature, same result.
Keeps my butter fresh for as long as it takes me to go through it (never longer than 2 weeks) with periodic water changes. When I've cleared out the bell, I just toss everything in the dishwasher to get cleaned and sanitized. No problems with mold, mildew, or anything else. Pretty simple design, there's not much to go wrong.
I have been using it for about a month now, changing the water every 3-4 days, and haven't seen any mold or wasted any butter. I'm a happy person about this, to be sure.I wasn't particularly obsessed with butter before, but now I bake and eat toast more just because I can. It holds an entire stick of butter, which is more than enough. Great price, too. Hoorays all around!
If you love toast, you'll love a butter keeper. After a few days of using a butter keeper, I could never go back to using cold butter straight out of the fridge again. This product does exactly what it claims to -- it keeps butter fresh and soft enough to spread easily. I change the water everyday and have never had any problem with mold. Get a butter keeper -- you'll never tear your toast again!
My first Butter Bell ever. The product arrived promptly and unbroken. The lid is not perfectly sealed but I don't think that matters as the seal is between the butter and water. I did add some salt to the water as I understand salt prevents mold. I do  change the water every couple of days. I also don't like seeing bread crumbs on any butter so I kinda clean the butter before placing it back in the bell so the water stays cleaner too.
Answer: As some have mentioned you can use salted butter, but the preferred method to help from the start is to add a small amount of salt to the water itself.