But then I come in and undercut you, by selling 10 cows at $75. So you bought the bullet and bought my 6 cows at 75.
Others see the feeding frenzy that is occurring and start selling their cows at 74. So now I am forced to try to sell my remaining 4 cows at 74, they see my offer, and undercut me by dropping ask to 73.
Buyers of cows see the ask dropping, so they cut their bids to 70, and sellers follow suit, adding more and more cows at lower and lower asks. So I sell my 4 remaining cows at 70.
Now nooby is going "fuck!" and decides to buy double the amount of cows he bought at 75 at 70.
Now he has a DCA cost of 72.50 per cow, hoping to sell the cows later above that. Then I decide I want to buy back my 4 cows I sold at a discount 70, and sell them back at 72.5, and you get the idea.
The more activity you have in an instrument, the lower the bid/ask spread becomes, because more and more competition means more volume, and lower spreads.
It is the same with stocks, except instead of trading a physical thing, yer trading shares.