What's stopping me "day-trade" mutual funds?

External question — Personal Finance Stack Exchange Author: Cloud Original post: https://money.stackexchange.com/questions/135404 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. I invest monthly into several funds, such as this one. Last week, I noticed a new fund I'd just invested a small amount into ($100) had grown by 12% in one week (now worth $112). I investigated the fund and it's climbing rapidly, so it's possible the trend will continue for a while. This got me thinking, what's stopping me from investing, say $10,000 this week, then if next week it's climbed by 12%, I'll have $11,200 in the fund. At that point I can sell $10,000 of shares from the fund, put the cash back into my bank account, and now I have $1,200 of "free" money invested into the fund. Are there any obvious downsides to doing this? Transaction fees are very low on my platform and $10k is only about 30% of my available cash, so I can afford to tie it up for a while.