There are two skits that feature them from the early 70's. They're Martians and they land on earth and explore our world.
In one skit they find a telephone, and according to their little red book, they try communicating by making various animal sounds (cow, cat, chicken) but to no avail. Finally the phone rings, and they answer back "BRRRRRRRRRIIIINNNGGGG!!!!!!" and are happy to make contact with an earthling.
In the second one they find a grandfather clock, and believe it must be an earth person since it's tall, has a face, and hands. They hear the clock ticking, and think that's how it talks. Then the clock strikes nine, and the loud bells scare the Martians off.
There's also an impossible-to-find clip of Bob singing by the brick wall, and the Martians come in and join him.
Many years later, there's a clip of the Martians coming into Bert and Ernie's apartment, and Bert offers them oatmeal.
It's something how these characters are remembered by so many, and they only appeared in a handful of clips- although the first two clips were in heavy rotation for years