pour milk into a large (12oz) glass, add chocolate syrup
open a fresh small bottle of seltzer. Canada Dry makes a 10 oz. size glass bottle that is perfect because it can be quickly resealed, preserving leftover seltzer for a short time.
the egg cream is best made with fresh seltzer. If you use flat seltzer it's just diluted chocolate milk.
take a long spoon or a teaspoon and briskly stir the milk and syrup while quickly pouring about half the bottle of seltzer into the glass.
the milk will foam up, so stop pouring and stirring before it overflows.