In the early days of rocket technology, about 100 years or so ago, some critics claimed that rocket propulsion in outer space would never work

In the early days of rocket technology, about 100 years or so ago, some critics claimed that rocket propulsion in outer space would never work "because in outer space there is no air for the rocket to push off against". Well, as we now know, rocket propulsion in outer space does work, and those critics were completely wrong! But why? Explain clearly.