How can stocks change price hours after close of the market?

after hours trading.

I understand that trading after hours is called "after hours trading", but if its going on, why cant I do it?

Easy but dangerous, as I know from personal experience :-(( If you trade online at your broker's or the other sites, such as Chas Schwab, you can get into huge trouble and do it at any time of day or night. That influences stock price. Transferring funds from money market or bank is made - surprise, surprise-very simple.. I had a MM at Vanguard and bought several tech stocks the day they hit their highest price. Caveat.....

After hours trading depends on one's broker and the type of stocks one wants to trade... Check in with your brokerage firm.

Google "onlne stock trading" and then use restraint..Tie yourself to the mast, if necessary.
The firms make it SO easy. From one who knows, sadly.