Swedish teenage climate activist Greta Thunberg was forced to defend herself in a Twitter spat with German railway service Deutsche Bahn (DB).
The 16-year-old provoked a testy reaction from Germany's oft-maligned rail operator when she tweeted from a DB train on her journey home from the COP25 climate summit in Madrid.
"Traveling on overcrowded trains through Germany. And I'm finally on my way home!" she wrote, above a picture of herself sitting on the floor of a carriage surrounded by suitcases.
DB responded testily, pointing out that Greta had travelled in first class during her journey through Germany.
"Dear Greta, thanks for supporting us railway workers in our fight against climate change. We are happy that you travelled with us on the ICE 174 on Saturday," tweeted DB's official account in German.
"It would have been nicer if you had also mentioned the friendly and competent manner in which you were treated by staff at your seat in first class," they added.
The company then followed up the tweets with a press release, in which they added that Greta's travel companions "were sitting in first class from Frankfurt onwards".
Greta hit back on Twitter, claiming she had found a seat at a stop after Frankfurt, which she said was "no problem".
Traveling on overcrowded trains through Germany. And I'm finally on my way home! pic.twitter.com/ssfLCPsR8o— Greta Thunberg (@GretaThunberg) December 14, 2019
"Our train from Basel was taken out of traffic. So we sat on the floor of two different trains. After Goettingen I got a seat," wrote the 16-year-old.
"This is no problem of course and I never said it was.
"Overcrowded trains is a great sign because it means the demand for train travel is high!"
Liebe #Greta, danke, dass Du uns Eisenbahner im Kampf gegen den Klimawandel unterstützt! Wir haben uns gefreut, dass Du am Samstag mit uns im ICE 74 unterwegs warst. Und das mit 100 Prozent Ökostrom. 1/2— Deutsche Bahn AG (@DB_Presse) December 15, 2019
Noch schöner wäre es gewesen, wenn Du zusätzlich auch berichtet hättest, wie freundlich und kompetent Du von unserem Team an Deinem Sitzplatz in der Ersten Klasse betreut worden bist. #Greta 2/2— Deutsche Bahn AG (@DB_Presse) December 15, 2019
Her journey through Germany was the last leg of a continent-spanning trip, which began when she travelled to New York on a low-emissions yacht last August.
The 16-year-old activist started a worldwide movement last year when she began to strike from school every Friday to protest government inaction on climate change.
Refusing to fly, she has twice crossed the Atlantic by boat in recent months in order to speak at a UN climate summit in New York and the hastily rearranged COP25 summit in Madrid.
Her online run-in with Deutsche Bahn comes just days after a high-profile Twitter clash with US President Donald Trump.
After Greta was named TIME magazine's person of the year, Mr Trump tweeted that she had "anger management problems" and told her to "chill".
Greta responded by changing her Twitter bio to include the words "currently chilling".