So there was a man and wife,

The man left for work everyday at 9am using the bus. His wife, who was cheating on him, had her lover come to her house every day after her husband left. One day however, her lover came over and they were getting it on when she heard the door start to open. She panics and tells the lover "it's my husband! Quick hide under the bed!" And he does. The husband walks into the bedroom, she says "I thought you had left for work already" and he explains that he missed his bus. He then says "I heard another voice here when I was coming in, where you with someone?" She denies and says "oh it must have just been an echo in the room". "Oh an echo huh? Let's hear the echo" so he shouts "ECHO!". The lover under the bed plays along in order not to get caught, so he shouts back softer "echo..echo..echo". The husband says "that's weird" and shouts "ONE, TWO!"
The lover echoes back "one two! One two..." The husband is starting to get convinced but decides to test one last time so he shouts "ONE PLUS ONE!"
And the lover echoes back "two..two.."