Seltzer or club soda,
Start by putting about 1 to 1-1/2" milk in a tall glass. Add about the same amount of chocolate syrup. Add the seltzer and mix vigorously until the ingredients are combine. If you've done it correctly, there will be about 2" of white 'cream' at the top of the glass.
An egg cream is never taken through a straw. You should always drink it from the glass!