I just realised that 15€ earned and invested no has fair probability of growing into 32€ (42.56€ before inflation correction) in 15 years.
I thought I had a thorough grasp of the concept of compound interest. While that was true in a mathematical sense, I had until failed to see many of the very obvious real life consequences of that. Just mucking about with a spreadsheet and FI / ERE goals made this suddenly click for me.
This enables all kinds of funny math. A FI / ERE example. I plan to start cycling to work soon. For under four hours, I'll get paid 32.16€. That's about 8.5€/hour. But if investment portfolio performance stays somewhat in line with historical averages, my one hour effort now will be worth 18.18€/hour in 15 year, or 23.43€/hour in 20 years. While cycling! Totally worth it!