"Mädchen, steigt ein!"
@Christian has it all right!
And yes, the German language has several compound verbs and we are working on better dictionary hints for them. Until that's solved, you should only rarely see compound verbs in our sentences. However, you can always check if a preposition is following a conjugated verb and try it out as a prefix to that verb to identify possible compound verbs.
For example, "Sie steigt ein." where "ein" (in) is the preposition that is part of the compound verb "einsteigen" (to get in). Or for example "Er geht aus." where "aus" (out) is the preposition that is part of the compound verb "ausgehen" (go out). Or lastly, "Meine Eltern geben im Urlaub Geld aus." where aus" (out) is the preposition that is part of the compound verb/expression "Geld ausgeben" (to spend money) - and the prepositions comes all the way at the end.
"einsteigen" is a separable verb. Duolingo's dictionary doesn't recognise these verbs as one unit. You'll need to use a proper dictionary. I recommend http://www.pons.eu/
Some resources on separable verbs: