In the aftermath of the oxygen tank explosion in the Apollo 13 Service Module, the Command Module was largely powered down to preserve it for re-entry and the LM served as a lifeboat. Among other things, the LM Environmmental Control System would have to keep carbon dioxide levels at acceptable levels for the remainder of the mission. At the time of the explosion there were two lithium hydroxide (LiOH) canisters - a primary and a secondary - in the LM ECS, which, together, had been designed to handle the carbon dioxide output of two people for about 30 hours. What was needed was a means of dealing with the output of three people for at least four days. Additional LM canisters were hopelessly out of reach in the MESA. There were plenty of CM ECS canisters, but they wouldn't fit in LM ECS. Houston went to work devising a means of drawing cabin air through a CM canister into the ECS using only materials available in the two cabins.