You’d think pouring a bucket of ice over your head to raise money and awareness for charity would be a simple task, in logistical terms at least. When you’re Bill Gates, though, there’s no situation that doesn’t lend itself to a bit of engineering.

For those who’ve somehow missed it, the challenge is to raise awareness for the ALS Association, which funds research into Amyotrophic lateral sclerosis, also known as motor neuron disease or Lou Gehrig’s disease. Those challenged are requested to either take the ice bath or donate $100. If you take the ice, the informal rules say that if you then donate at least $10 yourself, you can challenge three other people. Between that viral element and donations from celebrities’ fans has helped raised at least $13 million in three weeks.