The stock just gave investors a solid reason to buy.Shares of Waste Management (WM 6.10%) surged on Thursday and were up 5.6% at their highest point in trading during the day through 1:30 p.m. ET.With earnings season in full swing, you may have already guessed why the waste management stock rallied today -- it delivered solid numbers for its fourth quarter and full year. What's important to know, though, is why Waste Management's latest numbers should encourage you to buy the stock now.Solid numbers from a ...