Training Grounds, Grim Monolith, and Karn, Silver Golem

Prerequisites

• Training Grounds on the battlefield
• Grim Monolith on the battlefield
• Karn, Silver Golem on the battlefield
• Grim Monolith does not have summoning sickness

Steps

1. Activate Grim Monolith's first ability by tapping it, adding
2. Activate Karn by paying , causing Grim Monolith to become an artifact creature until end of turn
3. Activate Grim Monolith's last ability by paying , untapping it
4. Repeat step 1 and 3

Results

• Infinite colorless mana