A first born child gets to enjoy his mobile peacefully.
When you are the second born child, you have to enjoy your mobile with your older brother all up in your bizness, turning the gosh darn thing on and off and smacking at it all rambunctious-like, nearing trampling you in the process.
The other difference- with baby #1, Mommy used the 20 minutes of mobile time to put on make-up or eat lunch. With baby #2, Mommy has to spend the 20 minutes listening to kid #1 scream "Up! Night Night!!!" until she lets him get in the crib with his brother and then has to stand right there making sure baby #2 doesn't end up with cracked ribs or missing an eye from big brother's attempts to smack the heck out of the mobile.
FYI - the reason Joseph is clad in just a diaper with wet hair is because mobile time happened after I attempted a shower. A shower that ended with Joseph in with me and an entire $25 bottle of Aveda face wash dumped into the tub. The mobile was supposed to keep Conrad happy while I clothed Joseph...but clearly that didn't work out in my favor.