How do you interpret "it" from "A bit of a laugh"?

[Bhikkhu1991a]

Hello,

What are the definitions of the following underlined “it”, please? What are their antecedents, for example?

Here are the excerpts from English.test.net--the Newsletter “A bit of a laugh” dated November 14, 2008 http://www.english-test.net/newsletter/a-bit-of-a-laugh-175.html

1. Of course you can laugh so much that you simply can't stop. You know what it's like, it's catching. (Paragraph 4, line 2)

Answer:

2. You want to laugh but you can't and so you keep it to yourself and laugh up your sleeve — I should imagine if you tried doing this, it could be very awkward! (Paragraph 4, line 8 )
Answer:

I hope to hear from you soon. Thank you so much.

Best wishes.
:) :)

[Kanjikook]

1. Of course you can laugh so much that you simply can't stop. You know what it [laughing so much] is like, it [laughing so much] is catching. (Paragraph 4, line 2)

2. You want to laugh [out loud] but you can't and so you keep it [the laugh] to yourself and laugh up your sleeve — I should imagine if you tried doing this, it [laughing up your sleeve] could be very awkward! (Paragraph 4, line 8 )

Hope that helps!

[Bhikkhu1991a]

Hello Kanjikook,

I greatly appreciate your encouraging response.

Thank you so much.

Best wishes.