I usually compare capital to a fluid in tanker truck trailer. If you leave the tank without any device to prevent it, at the first turn, even taken at medium-speed, the transfer of liquid inside the tank will create a massive imbalance which will topple the trailer. So, usually tank trailers are fitted with inside perforated walls to slow the fluid movement. This can be compared to capital controls.

Another way to prevent the problem is to increase the viscosity of the fluid, which could be a good metaphor for taxes on transactions...

"Dieu se rit des hommes qui se plaignent des conséquences alors qu'ils en chérissent les causes" Jacques-Bénigne Bossuet