I've found this DUPLO brick:
The red part can be pushed in and snaps out when the button on the top is pressed:
Any idea which set this is from or what it could be used for?
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Looks like the Duplo Car Launcher and according to Bricklink this appears in at least 4 different sets: http://www.bricklink.com/catalogItemIn.asp?P=31080c01&in=S