Most oscilloscopes have a built-in square wave generator with an output somewhere on the front panel. This is used to tune the capacitance of your probes, which usually have a small tuning knob in them somewhere, to make the squarest-looking square wave.
Looking at a photo of the 2225, it appears that the output is labelled "PROBE ADJUST". Connect the probe to this and the ground to a suitable ground point, such as the outer rim of the EXT INPUT BNC connector, if you have an alligator clip for a ground.