Ah Yahoo! Messenger. The fun I had when it first came out. With the first desktop client, you could change font colour and embolden text. Also it had a default width, that almost nobody changed. User names were shown in the window in coloured text, which meant you could type a message, and then at natural line breaks change the colour of the text, so in one message you could construct what looked like an entire message thread. This trick could only be made to work if the user hadn't resized the window, because you had to rely on line breaks of your message recipients window being at exactly the same point as your own window. Also you had to reverse the colours of the usernames so they would match what the message recipient would see in his window. But that was fine. 90% of the time the window width was the default and the trick would work.
You could prepare the conversation thread in advance and then when the "away" or indicator appeared, paste it into the message window.
I played many practical jokes. Once writing out a message thread that looked something like the following.
Younger readers, please note this was before "sick" meant "good"
"Jack: Wow, what a night I had last night"
"Fred: Hey tell me man, did you go out?"
"Jack: Yep, I went to Heaven. Boy it feels good to be out the closet."
"Fred: But what about Jane? Have you told her yet?"
"Jack: Not yet but I will. I want to lose my butt virginity first, Just to be sure I enjoy it as much as oral"
"Fred: Shit man. Too much information. And the kids should also know - but not that much detail (obviously)"
"Jack: Not yet dude. Too young. Plenty of time to tell them later"
"Fred: Hey cool Jack, I'm glad your finding yourself. You can be my pal that tells me about the best moisturiser for men. I'd like to get some but have always been too embarrassed to ask"
[this was when the whole metrosexual thing was beginning]
Then you just had to leave the fake conversation for a while. After a while the "active" indicator would show, and you could just imagine furious typing.
"Fred: Yes Jack"
"Jack: That message conversation we just had. It wasn't me."
"Fred: Hey Jack, no worries, Cool with me. Be who you want to be."
"Jack: No you don't understand, that wasn't me"
"Fred: Look Jack, I'm sure it takes some getting used to. It's cool man, breath. I'm here for you. Just promise me you will be straight with Jane dude (as in truthful, not not gay, as that's what she needs to know). She's a lovely gal and you owe it too her."
Oh the fun.