Control the goo with the arrow keys or WASD keys. Eat everything smaller than yourself. The arrow points to the largest nearby entity that you can eat. Avoid larger entities - some them will hurt you if you touch them. Some of them will even eat you!
Control a tiny ball of grey goo which eats everything around it. The more it eats, the bigger it gets! The goo eats anything that gets in its way: dirt, bacteria, bugs, mice, cats, cars, trees, houses... Everything! Soon the entire planet will be eaten by this crazy ball of goo. Fortunately you're on the goo side.