This cartoon looks at two meanings of let down.
Definitions of let down
Let down - When you let something down, you deflate it (let the air out of it).
"You have put too much air in your car tires. You should let them down a little."
"We let down the air mattress before we left the beach."
Let down - When you let somebody down, you disappoint them.
"You are coming to the party - you won't let me down, will you?"
"I felt like I let down my parents when I failed my exam."