Because unless she's in bed with Castle, Beckett is always armed. ;)
Bromance between Ryan and Esposito
Ryan or Esposito says "Check this out"
Esposito says "Yo"
.....10 push ups
Every time Beckett and Castle smile at each other or kiss
.....20 bicep curls
Someone drinks or makes coffee
.....15 lateral raises
Someone is interrogated at the precinct
We see the murder board
.....15 shoulder presses
We see Lanie, Gates, Alexis, or Martha
.....15 tricep pushups
A gun is drawn or shots are fired
.....30 second plank