Cooling Extremities

Terra Industries seemed to have too much daily volatility so I sold it this morning for a small profit. I bought an almost equal amount of Navistar (NAV) and discovered my feet had cooled for the market generally. Why am I risking money I need for money I don't need? I have a limit order to sell Nav as soon as it goes high enough to pay back the purchase price, the two ten dollar commissions, and lunch.