In class I discussed the Mice vs Owl model and generated a direction field on my computer. You can do this, too, for free.

Sage is a computer algebra system (read, fancier than a graphing calculator thingy you can use on your computer) that you can use online or download. Sage has a built in command for generating direction fields, or slope fields. Below is the code I used to generate the slope field you saw in class.

p,t = var('p,t');
plot_slope_field( .5*p - 450, (t,0,5), (p,800,1000))